Beachball,
You don't say what country you will be using it in, this makes a massive difference.
I would not use a UK one abroad, it will cost a fortune !
I have a BT dongle which i wangled out of them for free when i renewed my contract.
It is supplied with a Vodaphone PAYG sim, which you top up like you do any mobile,, by purchasing time, from outlets, cash machines , or by ringing a number.
Pop into a currys or any of the phone shops and have a chat.
Make sure you have a look at the coverage maps.
Some are also time based and others data useage.
With that in mind you need to have an idea what you are going to be using it for.
Checking emails once a day, checking the weather on the internet, or full blown file transfer, film watching extra.
I think it is safe to say that the 3 network is more data based, so there would be the starting point.
Which ever you get coverage is the same as mobile phone as they work off the same network, but unlike talking, data speeds can vary, even by turning your laptop round a bit.
Mat