Get an Apple iConsole.............best thing since sliced bread! (Whoops................wrong thread :P)
I am not a big games player, but what I do know is look at the type of games your kids (and you!) like playing and then see which console has the best games for their particular taste.
My little brother used to have an Xbox360 with loads and loads of games and now has a PS3, but to be honest he only got one because his friends got PS3's too - sheep really.
Slightly off topic, but I know on the Xbox 360 Forza Motorsport 2 has the complete Nürburgring (Nordschleife) circuit on it, but can anyone confirm if Grand Turismo 5 on the PS3 also has the complete track?