I'm planning on getting the HTC Desire on contract, and T-Mobile and 3 are my best options when it comes to affordable unlimited internet. On T-mobile, it'll cost me £25 a month, and on 3 it will be £30 (both with free handsets). I'm currently on 3 payg, and I'm quite happy with it. But since T-Mobile is cheaper, I'm tempted to switch to them. The only thing I'm concerned about is their 'fair use' policy for the internet. I'm not entirely sure whether I'm allowed to use IM.
I've checked their policy on their website. It sounds like only Android users are not prohibited from IMing, but other pay-monthly customers are

Here is a snippet of their policy:
Pay monthly plans which include internet on your phone
Remember that you can only use internet on your phone in the UK and you can’t use your phone as a modem or use internet on your phone for peer to peer file sharing, instant messaging or making internet phone calls. *Internet on your phone comes with a fair use policy of 1GB a month. We’ll monitor how much you send and receive each calendar month so that we can protect our network for all our customers.
Pay monthly plans which include internet on your phone on an Android phone
Remember that you can only use your internet on your phone Booster in the UK and you can't use your phone as a modem or use web'n'walk Plus for peer to peer file sharing, or making internet phone calls. *Internet on your phone comes with a fair use policy of 3GB a month. We'll monitor how much you send and receive each calendar month so that we can protect our network for all our internet on your customers.
I don't understand why they have a difference between the two.
The other thing I wanted to ask is whether T-Mobile is any good and worth switching to?