Xray has the best belt design and HPI has the best shaft design. The reason is that they are the lowest and most centralized of all the cars in the weight distribution department. I can definantly feel the balance of the saddle packs and the central motor in my Xray, but when I raced the Pro 4 I could get better traction because it had a lower CG.
As far as the shaft drive and centered batteries, The car would need to use extra gears or 2 CVD's to connect the drivetrain. This would cause extra friction and reduce the useful output relative to the total input, a.k.a. Efficiency.
The Xray FK is the best overall design because you get centralized batteries, motor, and drivetrain. The only downfall is the higher CG of the rotating drivetrain.