You can't beat the Oral B. Don't buy the cheapest, as it is underpowered. Go for the Oral B 2000. Any higher than that and you are paying for useless extras.
The head-shape is personal - Oral B have a round head which rotates, Sonicare have a convention shaped head which vibrates; the choice is yours. Apparently "experts" think that the massage effect on the gums is as important as the teeth cleaning aspect; some brushes have a built-in timer which encourages you to brush for about 2 mins in 4 x 30 second sessions (front inside, front outside etc) and I feel it's worth buying a brush with that option.