MiFi - yes very good, converts 3G to wifi, much better than a 3G dongle. But the downside (as with the dongle) is limited data, high over-run charges and very limited performance. In my area I couldn't connect reliably daytime, had to do overnight to get any speed. The sim is interchangeable with any data-enabled phone on the same network, so no need to get a new sim. Can use anywhere in the uk that has a 3G signal.
BT - no probs here. You may get cheaper deals elsewhere but without the flexibility of being able to connect to any enabled BT Hub anywhere in the uk.