The ofna diff and hotbodies diff are the same size ( cup & bearings ), though there are a few 1/8 diffs that are slightly smaller- cant remember off-hand so its best to stick with the ofna of HB ones; prices vary, but not by a great amount- both are great diffs
Novak is good when it works, a royal pain in the anus when it doesnt; just today some guy on another forum has had to send his new hvmaxx system back twice because it broke down for no apparent reason ( locked rotor, that wasnt locked... ).
Its 2/3 the price of a new MMM system though, thats the main advantage ( along with not being too much power for a young child to handle ), although you could always look at the hobbywing escs and motors, or even a brushed setup ( kershaw designs 650 motors and an evx2 esc or something along those lines ). Its all personal choice really- you can tame down the MMM, get a beefy brushed setup, or use a hvmaxx and save money vs the MMM....