Definitely don't want the shaft down the side, the gyroscopic forces at speed would make the car go in circles, not to mention the nasty torque steer...
I'm partial the the Yok SD drivetrain... Diffs come out with 4 screws and no disassembly or the rest of the car... and the same arms on all 4 corners so you don't need as many spares... Nylon for the arms and mounts... strong, light and just enough flex but not brittle...
Spur gear directly mounted to the center shaft to cut down on rotating mass... With an ingenious way of getting the shaft in and out quick to facilitate spur gear changes...