Huh. I'm not sure I even knew there were two source port projects. I got the game, then just tried Rebirth because it was someones first suggestion and stuck with it.
I'll give XL a try and see if I like it better. Rebirth has been fine so far, so I'm interested to see the differences.

Some update on D2X-XL after my being absent from here for quite a while (I had a lot to do, trust me! ;).
The D2X-XL code base has received a massive overhaul, with significant parts of it having been rewritten in C++, using many of its features like virtualization, operator overloading and template classes that tremendously help in checking code and data integrity (arrays for example are now implemented using template classes, allowing to check array accesses).
One side effect of this code improvement was that the memory management has been improved, so that D2X-XL now usually requires about 40% less memory to run.
As if that wasn't enough, D2X-XL now offers complete modding support for Descent 1 and 2. That means that mission authors can easily mod every aspect of their mission, be it robot models and behaviour, textures, sounds and music, w/o affecting the original game data or requiring the user to deal with them.
Apart from these big changes, D2X-XL has received a lot of polish: The cockpit for example now looks better than ever, and there have been tons of little tweaks that just subtly add to a better gaming experience.
A reminder for all you die hards: You can turn D2X-XL into an almost identical clone of the original Descent 2 with a single command line switch: "-pureD2" will do the trick for you.
D2X-XL also is for the Descent 1 lovers, as it supports playing Descent 1 levels with the original textures, sounds, robots including the Descent 1 AI (yes, you can have D2X-XL use the Descent 1 and not the Descent 2 AI for Descent 1 missions). ;)
The only backdraw, if you want to call it so, is that a complete D2X-XL installation has over 4 GB. Actually that's another plus though, as the bulk of this data consists of high resolution textures and models and its download is completely optional. If you just want to add D2X-XL to your existing Descent installation, you will have to download less than 15 MB.
A public beta test is currently in progress and almost finished, so prepare for Descent with a new D2X-XL that is better and easier to handle than ever! :)

Will the Mac version be updated as well?

I hope so. simx, the Mac coder, is working on and off on it.

Well, I tried D2X-XL. I had tremendous issues with sound loops and crashing with the release version. Tried the beta from the forums there and some things seemed better, then it started doing the same thing.
It's -gorgeous-, don't get me wrong. It'd be my preferred way to experience D1/D2.
But given its total lack of stability, I couldn't deal with it.
Got Rebirth for both, plugged it in, just blew through a mission, which is NOT something D2X-XL would let me do. I'm a fan of eye candy much as the next guy, but gameplay and stability come first. Rebirth does it, D2X-XL doesn't. That's my $0.02.
If they ever get D2X-XL stable enough that it's not crashing like a sorority girl filled with beer behind the wheel of daddy's car, I'll give it another go. :) Till then, Rebirth it is!

It's a beta. Don't expect it to work 100% flawless.
D2X-XL 1.14 beta 19 is working very nicely for me both on WinXP and Linux, and the bug reports for beta 18 have been few and easy to fix.
I suppose that most of your problems are due to your system and setup of d2x-xl. You just need to have missed out on having to install some new hires textures required by D2X-XL, as so far I have only mentioned that in the beta thread on my forum, and boom - there goes your game. Don't blame me for that. ;) I have seen so many complaints like yours, and in the overwhelming majority of problems with d2x-xl were caused by its user and not by the program. I am also asking myself why you are giving that kind of feedback here and not in my forum or the D2X-XL trackers on Sourceforge.net.
Of course you are free to use any program or Descent 2 version you like.

Where would one find all these extra goodies? I've looked at the D2X-XL website and can only find the 15MB exe.

Woohoo! D2X-XL version 1.14 is out! Check it out, a whole bunch of bug fixes and enhancements. Also, all extras (hi res stuff) can be found under downloads on the side menu.

Found it, the layout and structure of that page really sucks.

And a new OSX version! Looks great. And your April fools joke was one of the best I saw all day.

Thank you very much for honoring the tremendous effort I and others are putting in this free game and game content offers with such a respectful and grateful compliment. You are certainly one of the finest samples of mankind.
BoxOfSnoo,
thanks. Have a lot of fun with it. :)

If you put your efforts on par with the look and layout of that site then you coding skills are as well developed as your ability to take critisism and the without any doubt best argument to stay well clear of D2X and use Rebirth.

Ah, I understand: You are a Rebirth fanboi on a mission to insult me.
How it must hurt people like you to see others accomplish things they cannot even dream of. Envy is a sting painfully piercing the soul.
Well, the good news for you is that your mission to make a fool out of yourself here has been successful.
The bad news is that you cannot blame me for your intelligence being on par with your social skills; and if navigating through my web site already has been overstraining you, you are certainly not in a position to judge my coding abilties.
Good bye, and keep staying well clear of D2X-XL.

Karx, calm down a bit.
I am a Rebirth fan because I like straight D1 and D2 without having to reorganize files and folders to get it started in nostalgia mode (or turn off all the features so I can get it down to a merely texture filtered version of the game); and I prefer the D1 robot AI to the D2 robot AI when playing D1. Plus, I use OpenSuSE, Mac OSX Tiger, and Vista Ultimate, and Rebirth seems to work better. That's just me, though. That said, I am planning on downloading and using XL for other missions that require it.
You're an absolutely fantastic programmer, and have conquered the Descent engine in ways many others could not. I've seen your work with XL, DLE-XP, and even your missions, and I never cease to be amazed. But Jema, while not exactly being understanding, was making a point - the layout of your website is indeed difficult to navigate. Perhaps you could consider redesigning it in the future to make it more accessible?
Seek advice, not validation.
Otherwise, great work, as you continue to amaze me with your skills and talents.
-D2Junkie

D2Junkie,
the tone makes all the difference, and there is no need on my side to calm down.
If jema had wasted 10 seconds on considering how much work I am already investing in D2X-XL and the stuff around it, and had some decent attitude, he'd have chosen a less provocative way to express his criticism.
I'd have nothing against a remake of my website, but I am not gonna do it. I also think that while it may be a bit colorful, it's actually not that hard to navigate - once you've discovered the side scrolling menu (and hey, it has a label "Side Menu"). People simply aren't used to it. The side menu offers all important links, and to get D2X-XL or DLE-XP, you just need to click on their logos on the home page. It doesn't get much simpler.
As far as Rebirth and XL go: To each his own. jema has reaped my reaction for the way he was talking to me in the first place, not because he may prefer Rebirth. There's a difference between saying "Hey, XL is shit and Rebirth rules" and "Hey, I just like Rebirth better for its simplicity".
As far as simplicity goes: D2X-XL has so much more data that it just has to use subfolders. Imo a small price that's to be paid here for a greatly enhanced Descent 2 experience (provided you like the enhancements - and I just cannot imagine why one wouldn't, particularly since v1.14 is out, which has been enhanced in every respect, including the lighting). Just look at the possibility to organize missions in subfolders D2X-XL offers. What is better: To sort your missions in subfolders by author or theme, or have one flat mission folder containing 1000 files? I also wonder what is making you think you'd have to reorganize files or folders to start D2X-XL in nostalgia mode: Once you've set everything up, you won't have to reorganize anything for either nostalgia mode or new style. Btw, I am using D2X-XL on various Open SUSE and Windows XP flavors, and it works fabulously there.
From what you are writing I can see that you haven't tried D2X-XL for quite a while. 1.14 is tremendously stabler, faster and simpler than previous versions. The menus have been massively simplified and D2X-XL detects a lot of things automatically. Above that you have the new modding support that is available for every Descent 1 and Descent 2 mission. The D1 AI was btw. available in 1.13 already. If you find the time, read the last three or four worklog entries on my web site to find out more.