We have an Oreck XL which we bought because of our dog. It's seven pounds, and we have been very happy with it. The beater bar spins at about twice the rate of the typical vac and the bristles are thick.
The Kirbys are a great vaccum also but heavy, however, the extra weight helps on thick carpet.
If you go with either of these brands or something of similar top quality, the motor will last darn near forever so buying a used model on ebay is a great alternative, plus you don't have some punk a** kid hard closing you on your own turf. A high quality used unit will work better than an average new one.
Belts, bags, and beater bars are relatively cheap.