I've got a Cube and Xbox. The cube has more kid friendly games, but both PS2 and Xbox are "better" gaming systems, with better graphics, faster processors, etc. I love the Xbox, but PS2 has more games, often gets games before the Xbox, and is compatible with PS1 games (and PS2 games will probably be able to be played on the PS3). I am not sure if the Cube games will be compatible with the next generation Nintendo system (but in the past, there hasn't been inter-system compatibility).
Finally - Gamecube is a lot cheaper, which probably ought to be considered.