With my O2 SIM I have used the Novatel Mifi 2352 and also the Huawei E585 (unlocked from Three) and both work well with similar performance.
The E585 is only single band UMTS 2100 whereas the Novatel is Tri-band 900/1900/2100. I thought this would make a difference in Central London but I have not seen any practical difference.
With the Novatel, if you connect it to a computer via USB, it switches off wifi. With the Huawei, if you connect via USB, you can still have 5 devices connected by wifi.
There is also the newer Huawei E586 appearing in the UK which does HSDPA+.