Like Ryhadar said, you don't need an electric if you practice perfect brushing technique, but electrics afford you more consistency - every dental hygienist/dentist I've been to recommend electric brushes. I've moved several times in the past few years and have been to many dental practices; it seems like brand recommendations vary based on preference and familiarity. To my knowledge, one brand isn't clinically better than the other.
I'm actually going to recommend one of the cheapest ones you can buy - http://www.amazon.com/gp/product/B002HW ... UTF8&psc=1
. It uses the same brush heads as Oral-B's most expensive models, so you get the same quality of equipment and relatively cheap replacements. It has a two-minute timer (it pulses when you're "done", so you don't over-brush), and that's it for the fancy features. I prefer it because you can use AA batteries - I stick a couple eneloop rechargables and swap them out when they get low. The expensive models with built-in Nimh/lithium ion battery packs are a pain to service (I had one), and the batteries wear out much quicker than you'd think.
If you're very conscientious about not running the battery flat and always have a spare outlet to charge, then the more expensive models might be better. But I use an electric because I'm forgetful and I don't want to think about things right before I go to sleep, so the cheapie AA model is better for me.