So, I'm absolutely going to buy Divinity Original Sin next week.

My options are:
a) I buy the retail version in a local store (which activates on Steam).
b) Steam. Just like a), just worse.
c) GOG.

To be honest, I'm not 100% sure if GOG is going to be my platform of choice this time. Don't get me wrong, this is still my favorite store. While I don't care too much about most Steam exclusive features (Steam cloud saving, achievements) as I rather get my games free of any DRM. I'm mainly worried about these two things:

1) Mod support: The Steam version comes with support for Steam workshop. Are most or even all of the mods on Steam workshop available elsewhere (like another modding site dedicated to Original Sin or the some forums)?
2) Co-op/Multiplayer: I want to play Original sin with another family member who owns the Steam version - and that's pretty much the problem if I'm going to buy the GOG version. Game updates usually appear much later on GOG than on Steam (yes, I know why this is happening). Disabling automatic updates on Steam might help.
Also, I'm stuck with a weird hybrid system of IPv6 and IPv4 (my ISP went out of IPv4 addresses), so I can't be perfectly sure if multiplayer is going to work via Direct IP or Virtual LAN.

And no, I can't go for the obvious solution: Buy the game on GOG AND on Steam. If I had the money, sure. Sadly I don't. ;) What do you think? Can you give me some feedback on those main concerns? This choice is almost driving me crazy...

You might want to wait to see what GOG Galaxy has to offer before buying. It's difficult to know how future mods will work in the GOG-version but multiplayer might be possible with Galaxy.

I cannot help you with that multiplayer question but I can make some guesses about the mod support thing:
The mods are created using the Divinity Engine and should work regardless of the version (GOG or Steam) of the game you have. As I understood Larian they want to make the mods available for everybody so I guess that there will be an option to download them from their page or a similar place in the future and that they won't be exclusively for Steam.

This will be resolved later on since Divinity: Original Sin will support GOG Galaxy once it is available (see the Status Update); Galaxy will allow GOG and Steam players to play online with equivalent integration features.

Let me hijack this thread just for a moment since i had some doubts of my own:

"Multiplayer notice: Multiplayer is only available via Direct IP or LAN."

Does this mean i'll have to give my ip to my friend (or use hamachi) so that we can play together? Is it easy to get the coop going? I'm asking this because most of the games where i have to do this (direct ip) give me headaches, something fails, need to open ports, configure the server so that is open, etc... In the other hand, this only happens with indie games that are in EA on Steam and is related with source engine...

Buy it on gog.

play through the game in single player

buy it on steam in 6 months-a year during a sale for steam workshop mods if you feel it's necessary then. >_>

Well, at any rate I don't really think there'd be much in the way for quality mods so shortly after a game's released so I don't really see it as a pressing need

eh I admit I didn't see that part.

not like it's really meant to be seen as serious advice.

This is not always a good thing. Sure, there might be good mods, but there's plenty of not so good mods as well. This is a matter of opinion, of course, but I prefer to play my games as they were intended to be; the only exceptions are bugfixes or stuff like that.

How is Mod Support bad thing? No one forces you or anyone else to use them. You don't like mods, that's fine but why prevent others from modding or enjoying mods? How is existense of mods harmfull for people who don't like mods or like to play vanilla version? How do existense of bad mods harm you if you don't use mods?

I would recommend trying a direct IP / virtual LAN connection with another game first, if at all possible. If you're able to connect this way, I'd recommend picking it up on GOG and having the other person disable automatic updates (though patches on GOG have only been on average about a day later than on Steam.) The decent mods will all end up on moddb or nexusmods eventually, and should work just as well with both the Steam and GOG versions.

Unless steam is your preferred store or client, then don't get it there.

I've given up on steam and won't support them anymore, regardless of what goodies they may offer me for free or otherwise. Yes that means certain features aren't available in my games, but at least i can play them without any issues...

So as far as mods go: http://www.nexusmods.com/divinityoriginalsin/?

As for Co-Op multiplayer, the only thing you're missing is online play. LAN and Direct IP will work regardless. Admittedly I'm unexperienced with ipv6 connections but there's a workaround for sure regarding that.

I wouldn't recommend Steam even if hell blows over.

To add something. Javier0889, have you ever tried using good mods? It extends the life of a game and can add amazing stuff for free.

Like Ganni1987 I'd recommend using that nexusmod link as theres usually heaps of mods for supported games and if the other games are anything to go by, there are usually a lot more exclusive mods on nexus, not available via steam workshop