The Sphere is brushless, the QC3 is brushed. Not as much tech goes into the brushed, therefore the lower price. Brushless motors will be easier to maintain in the long run, the motors are more efficent, too. Since everything is going brushless now days, I'd also have to say get a brushless speed controller.
Speed Merchant Rev7, Tekin, TQ Racing (wire), Team Tamale
RC Excitement - Buy where you race, support your local tracks.