Take this as a biased opinion if you want because I havn't really played the GBA version but I can only see the PS2 version as superior for all the extra things it adds. It's more-like the original in terms that it is 3D, there is voice acting and with the additions it just fits better in the series.

They didn't do the remake half-heartedly which is admirable, they had to get several new voice actors just to give some characters unique voices for the cut scenes. When voices are involved it adds a lot more emotion to a scene which is something that Kingdom hearts is great for.

Overall though the battle system frustrates me to death. Kingdom Hearts 1 had an amazing battle system but they decided to just ignore one of the best battle systems created because they needed to slow it down or something for an inferior system. This leads me into another frustration with Birth By Sleep being made on the useless PSP. I want a remake on the PS3 or something but I heard that they have no intention to for no good reason. I want to play Birth By sleep so badly but I don't want to waste money on an over-sized paperweight that forces you to rebuy movies and games if you want to play them in squinty version.