There are several things you need to consider before you make your purchase.
1. Price
An Xbox 360 with 250GB HDD will set you back around £180 and if you also want Kinect then you're talking about £250. This includes a headset and everything you need to get set-up, but if you're wanting to play online then you're going to have to purchase at least one gold subscription and those are between £25 and £40 a year depending upon where you get your subscription.
A PS3 with 500GB HDD will cost you around £200. It doesn't need a subscription in order to get online and if you pay for Playstation Plus, you get loads of free to use software each month - allowing you to build up a game library without having to spend a fortune on games. You lose access to these games should you ever cancel your subscription.
A Wii U with 32GB storage will cost you around £250. It also doesn't need a subscription to get online. It's got a very limited game catalogue so far and it's not looking like picking up any time in the near future. It is also tailored far more towards youngsters than either of the above consoles.
An Xbox One will cost you £429 when it is released on 22nd November. It will come with Kinect as standard. You will also require a subscription to play online.
A PS4 will cost you £349 when it is released on 29th November. It will also require a subscription to play online.
2. Reliability
The Xbox 360 had a terrible reliability record during it's first couple of years. I went through 6 Xbox 360s in that time frame. The PS3 - while not as bad as the Xbox 360 - also had it's teething problems and I went through a couple of those too.
The Xbox 360, PS3 and Wii U are now all fairly reliable, but as yet, no one knows how reliable the Xbox One and PS4 will be. Past experience has taught me that any new technology is liable to suffer a higher than normal failure rate.
3. Games
The Xbox 360 and PS3 share many of the same games and both consoles have a HUGE library already available. The games are easily obtainable and older games are available at very reasonable prices. Second hand copies can be picked up for peanuts. There are games for people of all ages.
The Wii U has a very small library of games and most of them are tailored towards the younger end of the spectrum. They're also quite expensive.
The PS4 and Xbox One obviously have no games available yet, they'll have a very small library available for the first 6 months and they'll also be the most expensive. The prices are likely to remain high for the foreseeable future.
There are a few random other things to think about.
The joypad on the 360 is significantly larger than the PS3 and when my niece was younger she found the 360 pad much harder to use.
Kinect is not hugely accurate, but it's still fun to use, but it requires a large room with relatively plain walls to function 'properly'.
The Xbox 360 and PS3 will still have games published for them for the foreseeable future, but new releases will start to trail off as the next couple of years pass.
I'm sure there are other things that I've forgotten to mention, but I've already typed the equivalent of a short story
