(coming from a Wii owner, so a bit biased, but oh well)
I'd say get a Wii over a PS3. If you already have an XBox 360, then you don't need a PS3. PS3 has barely any exclusive titles that XBox doesn't have. Plus, the Wii is $150 cheaper, and yes, it has free online like the PS3, but it's annoying, so stick with Live. The Wii games may seem childish, but they are really not. You look at them and think "There's no way I'd like this game, it looks like crap!" and then you play it and you love it. Take for example Animal Crossing. That game looks so childish but me and my friends have a ton of fun playing it online. The Wii games deserve a chance.