3g+ can be good to get you out of trouble or as a temporarily measure, biggest issues if its staying in the same place for 24 hours at a time is good signal and speed.
Worth doing your homework on best coverage, or trailing it first, considering most Networks give away payg Sims oline.
Ive tried some of those 3g Wireless Routers from Hong Kong without success, in my workshop which is in the middle of no where I use an old Sony Ericsson Satio by the window on constant charge with some MiFi software (Joikusoft) works really well, its left on 247, I can return after a few days and its still happily running.
On the speed side, Voice Paygo or Contract Sims will give you around 1.5 to 3.5 Mbs and proper Data Sims 3.5 to 7 Mbs Network depending, there is of course EE 4g but in my book its still early days for that one.
Nwtwork wise, on PAYG Sims, 3, T mobile and now Virgin give unlimited Data use for X amount per month with T Mobile being the cheapest at £5 a month or £20 per 6 months addons, shy away from the so called free 12 month iPhone Sims, they wont last the course if Data is hammered.
Broadband Sim wise best value is 3 or T Mobile (30 Day Contract), 3 gets expensive if you go over the FUP, where as T Mobile wont charge if over thee FUP but will restrict the downloading, normally Video formats iPlayers etc, but a quick Network reset normally gets around that.