I suggest Blizz-ABS as well. I've personally used most of the ABS's out there, and I can attest to the superiority of BlizzABS. In the past, there were downsides to using it (mainly graphical), but since 2.0, those tradeoff's have become nonexistent. The newest version of blizzABS can even be used to make a game with as much depth as Morrowind and Oblivion (some of the greatest RPGs ever made). Blizz ABS is lagfree, has the most features, is easy to use, has the simplest, most powerful graphic system, is well coded, has the most powerful enemy AI, and makes the most fun games (I've battled in XAS, neoABS, Mr. Moe's ABS, Near Fantastica's ABS, and evented ABS systems, but none of the battles are as enjoyable as blizzABS battles).
So yah, there are no tradeoffs I can think of with blizzABS, unless you want a Zelda game (in that case, you should use XAS, it's perfect for zelda games, but not for great RPGs)