Debs, I have tried most of them over the last few years and have ended up with two dongles lol, both are unlocked so I could just as easily just swap sims, but got both dongles on ebay cheap and find it easier/quicker to swap dongles.
Coverage, overall is much the same for most of them (Vodafone, T Mobile, O2, Orange) and that is good in most respects in and around most towns and along major routes, but most of them drop out into a GPRS/EDGE signal outside these areas, which means dial up speeds, groan.....
The best one for 3G coverage in my experience is the 3 network. I get 3g coverage from Dover to Balloch, certain areas on the East coast of England dont get a lot of 3g coverage, but 3 are working on it, on the west, down the M5 corridor has good coverage too, you really have to be off the beaten track before you lose their 3G, but dont go by their "coverage checker" I got the 3 dongle at first as they claimed I would get a signal at Loch Lomond, they lied. lol. However, 3 years later, there is 3G there now cos I kept pestering them, lol!
As for tarrifs, it all the usual choices of fone networks, but I have settled with 3 and Vodafone. With 3, if I am going to be downloading a lot, I'll top up with £25 for the month,which gives you 7GB of data to download, its not ideal as I can use that 7GB quite quickly at times,I could pay £30 a month with e.g. T Mobile and get unlimited downloads, but found at times that wherever I was at the time, only had GPRS with T Mobile so was no point even trying to download anything. Also, with the Vodafone, heres the odd bit, I will top up the Vodafone with £15, that gives you 1GB (3 give you 3GB for £15) but that 1GB can last longer then the 3GB from 3 :-? I listen to radio online, all day, everyday and can run out of data on 3 (3GB) in about two, nearly three weeks, before the month is out, yet the Vodafone carries on, the longest it lasted was nearly six weeks. Must mention here that the Vodafone doesnt have a monthly expiry time, it keeps going till the credit runs out, unlike the 3 which runs out monthly whether you have used your allowance or not and doesnt carry over.
Have rambled on, lol, so last couple of things, pay as you go for me is better as, with most networks, if you run over your monthly allowance when on a contract, they then charge you £1/1MB.... ouch, watch out, lol, best way you could check 3 coverage in your area, find somene with a phone on 3 in your area, esp a phone that can access t'interweb.
3.6 speeds are there but usually in bigger towns etc, 7.6 on its way to most areas, have experienced it and its Bloody fast!!! Way faster than a lot of home broadbands!
HTH