CFS2 for me. CFS3 has better graphics but I like flying CFS2 more. And sometimes I even fire up CFS1 just to do some dogfighting.

CFS2

Advantages:
1. Familiar open architecture
2. Great flexibility
3. Even though the Pac Th. is only available, you can add in the other theaters as well.
4. Easy on your computer in terms of processing power

Disadvantages:
1. Lower graphics - but since CFS2 is 5 yrs old, our computers are powerful enough to take on any enhancements.
2. It's old

CFS3

Adv.
1. Greater graphics
2. Dynamic campaign
3. Open architecture

Disadv.
1. Open architecture is totally UNfamiliar and new - thus there is no compatibility with all other addons.
2. Dynamic campaign takes up loads of processing power due to the extensive number of AI to manage.
3. Serious and careful tweaks are needed just to reduce the stutters - unless you're willing to fly at 5 FPS.
4. World is seriously confined to just the ETO. However, there is the Korean Campaign pack. But that's it.
5. Unfamiliar architecture also results in limited number of any type of addon.

Verdict:

Depends on how you use the two.