It's a difficult decision for anyone but yourself to make. Nintendo have never been a company to demand a high spec from their consoles, their games have always been aimed more at family entertainment (not including the very few cross-platform games).
If you enjoy more "intense" gaming, realistic graphics, racing and shooting at people, then the 360 will almost certainly be the console for you.
If you like cell-shading, puzzles and platformers better, it's likely you will prefer the play for the Wii.
I think it is going to be very similar to the comparison of a DS to a PSP. If you prefer the DS play style over the PSP then I'd say you'll probably prefer the Wii, and vice versa for the 360. I know that I love Mario and hate anything with lots of blood, so I'm really looking forward to getting our Wii, but I never touch the 360 because I just don't enjoy those sort of games.