Don't have either yet obviously. I have an old Xbox that I'm happy with. The Wii is cheap, but are there any real must have games or is it more of a gimick? I'm leaning toward the 360. I don't think I'll be unhappy with anything but the price.
If you can, wait til the fall. The summer is typically slow, and the picture will be much clearer by then.
PS3 - Not a whole lot to play right now. Probably will come down in price by fall.
360 - Many good games in the fall. Newer, cooler CPU coming. Best game selection, but the hardware is more fragile than grandma's china.
Wii - Solid hardware, not a whole lot of games yet. This will change by the end of the year, with many developers throwing money at it now that it's a hit.
I have a 360 and Wii, and i like them both. If you plan on playing with friends all the time at your house, Wii is more fun. If you plan on playing alone or online, 360 is better. I'll end up buying a ps3, but not until it comes down in price and actually has games worth playing. Until then, it's a $600 Blu-Ray player.