Marluxia I was definitely harder on GBA. In the GBA, he would attack much more often, and spammed Sleights like crazy.

Marluxia I in Re:CoM barely uses his cards at all and just seems really passive. However, his Sleights are quite harder to dodge in this one.

Plus there's also stuff in the GBA version that just made almost any boss harder; in GBA version, bosses don't get stunned as much when hit by attacks, so you can't Sonic Blade your way to victory as easily as in Re:CoM. Bosses in GBA version are also smart enough to break your Cures and Item cards; if you tried using a Cure or Item card in Re:CoM, you can almost always pull it off as long as you are out of their attacking distance, and that the boss does not have a stocked Sleight. In GBA CoM, if you tried to use Cure out of their attacking distance, the bosses quickly search through their deck to try and find a card with equal value to your card in order to break it.