i picked up the ps2 version. it's $10 cheaper and since the wii version is a port, it doesn't matter much.
i prefer "mlb 06: the show" for actual baseball gameplay. mlb power pros has decent hitting, but i don't like the fielding or especially the pitching. the fielding can bone you with which character is auto-targetted and the general change from the hitting view to the fielding view. you can get boned and easily start walking in the wrong direction, for example, if somebody hits a liner to the infield. i think the pitching could really, really use a meter mechanic.
i've played the living hell out of success mode, though. it reminds me of
princess maker 2, one of my favorite games of all time.