We have an XBOX 360, but have played the Wii and PS3 also. The PS3 is good b/c you can play bluray movies in it, and also play live, but that is about all. It doesn't have as many good game options, the games tend to lag and have worse graphics. If you look at the statistics of sales, PS3's have dropped b/c they aren't getting great reviews.
The wii is awesome, but outdated and is very limited with what it can do and most games (like if they want to play first person shooters like Halo or Call of duty) are very difficult to play. Also, you have to buy all the extra controllers, and young kids sometimes have a hard time using them, and the movements are sometimes hard to pick up. Our inlaws have a Wii and we are over the all the time, but don't play it very often b/c it just isn't that fun.
I would get the xbox360, you can stream netflix, play dvds, store family photos and play music, go live and play, and they even have a kinect game add on that is a lot like the Wii in that you move your body to play action and dance games, but you don't need controllers, b/c your body movements are picked up by sensors. It's pretty awesome and really fun for the family, both for the young children (even my 3 year old loves it), teens and even adults. really, the XBOX 360 has the best of all consoles if you get the kinect too for the wii fans.
info on kinect for the xbox 360:
http://www.xbox.com/en-US/kinect