My dentist told me to get an Oral B 3000 as they're supposed to be good for sensitive gums. It has a pressure thing in it that stops you brushing too hard [slows down if you scrub and press too hard]. It also has a 2 minute timer broken up every 30 seconds by a change in vibration, so that you cover the 4 areas of your mouth equally.

My missus got herself the model below that though which was a bit cheaper and does the same thing.

I've also heard the Sonics are the best and would have gone for one except for the reccommendation!