Xbox 360 is still a good platform, and still has great games being released. The Wii U will be a big jump over the standard Wii - but you will find that games for Nintendos machine tend to be more family orientated.
I would wait for either Xbox720 or PS4 towards the end of this year...
I agree with ulysses35's post. The Wii is definitely more kid/family oriented where the Xbox/Playstation consoles are typically more for the "hardcore" gamers. That being said, it really depends on the genres of games that you are looking to play.
There's nothing wrong with buying an Xbox now even if it is coming to the end of it's life cycle, in my opinion. In fact, you can probably score a console for a pretty good price (sans 250GB HDD) so you can put the rest of the money away for the next big thing when it arrives!