The Sonicare Elite is the best of the best if I recall... higher speed, higher frequency, etc.
I have a Sonicare Advance with built in toothpaste dispenser. It's great for traveling (battery lasts over 5 continuous hours of use... or about a month of regular use) between chargings and since it has the toothpaste inside the brush you don't need to bring separate stuff. Push a button while you brush and more toothpaste comes out and foams between your teeth... it's pretty nifty. Only place i know to get them OR the toothpaste is drugstore.com but it's not terribly pricy.
The original models are cheap, but were a first edition kinda thing... not that great and the heads needed replaced pretty often. Go with an Elite, Advance, or one of the other second gen models 🙂