It depends how much you intend to use it and what you intend to do! Viewing videos such as YouTube will gobble it up, sending and viewing emails uses very little.
Go to each providers website page and check their coverage - will it work where you want it to? I live in the middle of a big city and Vodafone doesn't work here.
Check the speeds too - none will be as fast as your home broadband, but some are faster than others.
Then you can check prices. Do you want 'pay as you go' or contract? If you only intend to use it occasionally PAYG may be for you.
You may not know but Virgin now provide mobile broadband, so check them out as well as TMobile, Vodafone and 3.
If you are in no rush, I suggest you install a broadband useage meter on your current pc. This will give you an idea of how much data you use in a typical day, week, month.
I find this to be good.
http://www.bandwidthmeter.co.uk/