Well, White Devil (couldn't resist) it would appear you've answered your own question. To inject my own views on the matter I would second most of the people here and suggest getting the Wii, as you'll then have have more of a diverse library of games available if you already own a PS3.

PS3 + Wii or 360 + Wii
(should be a rule :P)

Never PS3 + 360 since they are both hardcore consoles and pretty much the same games on each. Unless your a hardcore fps fan and hate adventure games I don't see there being a point in getting a 360. I'm not trying to be a biased Nintendo fan but it makes more sense if you get a Wii.

get 360 its better

Wii, only the best option

Wii for unique-ness and apparrantly its the cool thing to get.

Get 360 if you want the best online experience/ best games right now.

PS3 if you like Final Fantasy and Blu-Ray

If you own a PS3, the Wii is the obvious choice. The 360 and PS3 share a lot of games and that trend is likely to continue. Wii offers totally different stuff. It can be a tough choice between PS3 and 360, but if you're looking for a second console, Wii will definitely bring the most variety.

Considering you already have a PS3, I'd go with a Wii. PS3 and 360 titles will overlap enough that in the end you're purchase will be justified. And your wallet will be thankful.