The XBox is probably the safest bet all around. It has the biggest game library and the graphics capabilities are still pretty good. It's also the best option if you are interested in online gaming. Also the Kinect add-on has significantly energized the system so that it's probably got a couple years of life left in it.
The PS3 is going to be better as far as graphics and overall power. Plus you get the built in Blu-Ray player. Like has been mentioned already, however, the security of its online features is a little sketchy right now.
The Wii is on it's last legs as a system, which is why it's the cheapest. It's fine if you only want to play a few games here and there. However, it's getting replaced next year with the Wii U, so there aren't going to be many major releases left for the Wii. When it comes to technical specs, the Wii is a joke compared the XBox and especially the PS3. I would NOT buy a Wii at this point in time. Better to either get an XBox or a PS3, or just wait for the Wii U in 2012.
Also, if you're not particularly a fan of Mario or Pokemon games, then there is little reason to purchase a Nintendo system these days.
As of now, the XBox and PS3 are not expected to be updated until 2014 at the earliest. The Wii U should be out next year , but it is not really considered a next generation system, it's more like Nintendo playing catch up with Sony and Microsoft, because the Wii was such a woefully underpowered system.