Well, I recently finished S.T.A.L.K.E.R, Shadow of Chernobyl, and I was just amazed by the whole experience. The atmosphere, the gunplay, the kickass guitar solos by random people around campfires... I enjoyed the game greatly. Granted, there were parts of the game that annoyed me, mostly because I'm not really accustomed to games where I'm not supposed to act like a tank. I died an awful lot on STALKER difficulty, mostly as a result of my own carelessness, but overall I thought the game was a lot of fun.
Now I'm considering buying Clear Sky. Now, I'm feeling a little trepidation about this, because when Clear Sky first came out the reviews about it were rather mixed (and it still has a lower metascore than S.O.C), and I'm wondering how well it compares to the previous game. The Shadow of Chernobyl I played was enhanced with the latest patch and the S.T.A.L.K.E.R. Complete 2009 pack.
To the questions!
1. In terms of difficulty, how does CS compare to SOC? In SOC, my experience was that you had to fight smart, aim for the head (Screw center mass!), and have a good sense of the effective range of your weapons if you wanted to live, but living was possible.
2. In terms of scale, how does CS compare to SOC? SOC was not the largest sandbox game I've ever played, but it was still pretty big, enough that the idea of making a long journey was not something I took lightly. Then again, the map was designed in a roughly linear manner, and navigation was never really a problem.
3. In terms of bugginess, how does CS compare to SOC? I had a persistent glitch in my SOC game which would cause the entire game to freeze for a few seconds before resuming as if nothing had happened. I never did figure out why that happened, but it didn't disrupt the game overmuch.
4. In terms of interface, how does CS compare to SOC? It bothered me a little in SOC that I always had to open my inventory whenever I wanted to retrieve any item other than my primary/secondary weapons, grenades, binoculars, or a bolt. It would have been nice to be able to hotkey other items. It also would have been nice to have a larger carrying capacity...
5. In terms of atmosphere, how does CS compare to SOC? The Zone in SOC felt very real to me; if I had to imagine what a populated area might look like after being abandoned for 20 years, the Zone was just about perfect. Then again, even though the place was supposed to be sparse and post-apocalyptic, I never once got the feeling that I was alone in the wastes. If anything, I found myself wondering how the Zone ever became so crowded with the Ukrainian military patrolling the area and with the constant infighting between Stalkers...

the game isn't infront of me, but there were keyboard commands for all of those in SOC.

Skip Clear Sky altogether, get Call of Pripyat.
It has better atmosphere, interface, scale, difficulty, and less bugs.

One thing to keep in mind with Clear Sky is that it has both bugs and TAGES installation limits. Luckily there is a cracked version with the latest patch, so I recommend you download and use that when you buy the game.
One difference that I heard of between the two is how Clear Sky is more of a straightforward shooter and throws a lot of enemies at you. The atmosphere of SOC isn't quite there.
I haven't played it myself yet. But given the price and the cracked patched version, I've got it on my to buy list.
One other thing: the third STALKER is out soon. Unless you can find Clear Sky cheap it might be worth seeing how that one fares.

1) About the same, I think.
2) Again, about the same. Most of the areas in CS were reused maps from SoC. There's also fast travel (for a price) so if you have money, long trips are less of a problem.
3) That happens when you cram a lot of stuff into the blue storage boxes. Whenever you get close to one, it locks up for a bit while the contents load. Regarding CS, fully patched it's pretty stable. Crashes happen, but are rare, in my experience.
4) There are hotkeys in SoC for bandages and medkits too, though some mods remove them for "realism". Interface in CS is mostly the same. The inventory screen got a little tweaking, though I can't remember off hand what changed (besides the graphics). You can also increase your carrying capacity with suit upgrades and artifacts.
5) Since most places got reused, you already know what to expect from the architecture. Thanks to the faction wars system though, the zone appears to be even more heavily populated :/ There aren't really any underground missions (just one short one) so much of the horror aspect is gone. Rainstorms, sunrises, and blowouts are very nice looking.

It is simple.If you liked SoC, you will like CS.
I would give SoC 9/10 and CS 8/10.
CS has also great atmosphere, good graphics, some new good elements (artifacts have to be hunted with detectors, you can upgrade your weapons and armor..), some worse (faction warfare, it is better to ignore it..).
New locations are nice.Overall I would recommend it.I played 1.5.10 version and did not have any bugs personally.

Why ignore faction warfare?

Because it sometimes does not work correctly and anyway does not really fit with the atmosphere of the Zone...IMO of course.
I just remembered that I had one bug, and of course it was related to faction warfare..I was supposed to overtake garbage warehouse, I killed all bandits 4 times (!) but nobody ever helped me, and as soon as I stepped away from the location, the bandits respawned again.
So I went "fuck it" and continued with the game itself (which is great).