We have both the Wii and Xbox 360 with Kinect. When we are looking for something quick to play or something for the whole family, the Wii is used. When my 12 year old daughter has friends over and they want to play something, it also is the Wii. I like the Wii for many games, but I LOVE the XBox for some of their racing games, first person shooters (although I play most of those on a PC), and just about anything that can benefit from great graphics. My 12 year old also does have a few things for the XBox, and as a Harry Potter fan, she wants Lego Harry Potter years 5-7 and asked for it on the XBox since the graphics are better. She has years 1-4 for the Wii.
There are many positives and negatives about all systems. For quick, anyone can pick it up and play (including adults) the Wii is great. It is also great for party games. For something more substantial, look at something like the XBox or PS3. You can look at getting an XBox 360 with Kinect bundle at WalMart on Black Friday (or might be Thursday night) for $200 but you also get a $50 WalMart gift card. These will sell out fast, and WalMarts are famous for people grabbing things "off of the piles" 30-45 minutes before they officially go on sale.
