At present the top download speeds for mobile broadband are from vodafone at 7.2Mbps and 3 Mobile with 2.8Mbps broadband services , in reality though the actual speeds achievable just like home broadband rarely reach such heights , you should check the coverage area for you chosen provider to ensure you can get a good signal.
During December in the Portsmouth area my 3 mobile dongle took over an hour to download a 350k picture! It also sat for many minutes doing nothing, the network seems to be well overloaded. Reverted to phone line.
I got a 3 broadband modem and its brilliant hitting an avarage of 2.5 mb per second and its only 19.99 a month for 15 gb usage allowance
Using a 3 dongle in London, hitting a maximum around 2 mb which is pretty good….
Slows down a bit in the evening at peak time but still not bad.