You really need to explore the stock game, and then modded by LWAMI to see the differences. Improved models and weapon behavior are only the beginning.
The main difference of RA/AT is that other platforms become playable, including newly added ones in each mod. The ability to use nuclear weapons is another feature, although realistic nuclear explosions aren't modeled. RA also comes with some new missions tweaked or created specifically for RA.
__________________
 
|