Which is the best mobile broadband?

I am thinking of buying a pay as you go mobile broadband modem for when I go to uni, but unsure of the network.
My budget doesnt really matter, I'll put the staff discount to good use!
I have a few questions and queries first:
1.
The best network, who do people rate highly
2.
The best coverage and speed.
My uni choices are York, Huddersfield or Lincoln so the network will have to cover these areas.
3.
The easiest to top-up and best value for money
4.
Also, what happens if I top up for one month, does my download allowance get canceled in the next month, or is it like a mobile phone, where you can top up as much and as often as you want?
5.
Can I use the same modem with a desktop and a laptop?