I would go with the new. See, I'm all for "What if the DEVS instead of going all-new-gameplay in 3.x version just made a few adjustments in 2.x version with a new/old engine?". To make it fresh. To change the game (to make it better) instead of the gameplay.
I'm all for the new.
America's Army in its 2.00 version was released in November 2003, and last patched in April 2009. Considering that is a quite wide live support timelapse for a game, the game actually reached a certain maturity around 2.5 or maybe 2.6.
AA3 probably was in development at least since a couple years before, and by 2009 AA2 reached many limits due to its age. Also, AA2 has some technical limits that are bounded to the software age, and while the engine was probably a powerful tool back in 2003, as of today it's simply outdated in many ways (one of the most obvious ones is the higher load on cpu, due to a low use of the graphic processor power).
Lucky enough, what matters is not just how the game performs but how it actually feels when being played. So, i can't blame the Army for stopping developing AA2 (i would just blame them for stopping supporting it). Blaming over AA3 poor gameplay...well it's going to be 3 years old in July, something has changed, just not as deep as expected. Oh, they made the game actually playable, kudos for that [/sarcasm]
P.s. try take a look at AA3 poor characters animations, then look at the decent, AA2 ones. I can't find one animation that looks natural in AA3. The whole character body seems to be connected in a wierd way...