They are both great games. The campaign mode in the Red Baron games is what really sets it apart. It has a pretty good and challenging flight model (you can tear wings off, cause structural damage by putting yourself in too steep of a dive or high-g tight turns - WWI planes are fragile boxes of wood and canvas) and once upgraded with a glide wrapper (easy to do), the graphics are actually quite nice. But yeah, the campaign is where Red Baron 3D (and heck even Red Baron 1! :P) really shines.
I actually haven't experienced any bugs in Red Baron 3D (so far) running Win 7. However in order to get the 3D experience you do have to download a glide wrapper otherwise you only get the 2D graphics. I think Red Baron 3D is older (in terms of when it was added to GOG's catalogue) than some of the other titles here, which is why some of the more recently added titles that required Vodoo graphics cards come with a glide wrapper from GOG, but Red Baron 3D doesn't. In fact Zeckensack's wrapper which is what GOG uses works fine with Red Baron 3D. There are others that work as well (DGvoodoo, and another one from a Red Baron community member called CamelJockey).
There is actually still quite an active community for Red Baron 3D with the most recent major projects coming from the Red Baron Redux team at
http://redbaron3dofficialsite.com/mission.php. You can check the Red Baron forums either on that site or on GOG for other community mods or add-ons.
IL-2 is a great game too. However, the flight model (especially with all realism options turned on) is much harder than Red Baron's. For one thing, the planes themselves, being WWII planes, are more advanced and have simply many more options to tweak for in-flight performance. Both games can have unforgiving flight models on their highest realism settings, and so when I say that IL-2's is more so, this equivalent to Darth Vader saying that "the Emperor is not as forgiving as I am". They both have options to turn the realism down of course.
There are some GOG denizens who sometimes frequents both IL-2 and Red Baron forums. Wailwulf has a lot of expertise in both games and is probably a good person to whom to address your question. You can find him in either the Red Baron 3D or IL-2 forum. Bottom line: I don't think you'd be disappointed in either game. They are both great.
karacho: thank you, i think i now have the info i need, i just have to make up my mind if i put the dynamic campaign and immersion from red baron which both sounds great or the all arround excellency of sturmovik on top of my priority list
both games sound great and i really have a thing for dynamic campaigns, but chances are i still want to get sturmovik, its hard to decide though
seems I was too late :)