I say RE:Com is better. I've played both and the Play Station one is a lot better. I can't even figure out to put together the cards into those sleight things on the gameboy version. :(
Ok I've noticed some things that are different from KH2 and KH2FM. I KNOW that they're different games and that Final Mix has extra stuff but like one thing I noticed it that Roxas' hair is more blonde in KH2FM. Another thing
I noticed is that Sora's hair is darker in brownish color and that the bottom of his shoes are black in KH2FM and gray in KH2. The last thing I've noticed is that Roxas' face looks different in the cutscenes than from what they look like in KH2.
I think Xemnas pt 2 is the hardest but the three main difficult Organization XIII members for data matches are Vexen, Zexion, and Demyx. Axel is a little hard but only because the flame floor sucks up your HP really quickly.
I think Xion is definitely Kairy's nobody, isn't Xion in KH 358/2 days? Namine is a nobody but a special nobody that has the power to mess with memories.