ball diffs are smother, i have one in the front of my tt01
bevel gears are lockable and can hold a drift for longer and tunable with diff solutions and thickness of the substance you use to lock it with..

ball diffs are used for carpets and road racing as they offer smoother movement, but are not totaly lockable..., although they can be tightend fairly well..

ive ball diff my front, and locked rear in blue-tac...that is the best set-up i have found out to be so far..

i have tried ball diffs front and rear, just didnt do it for me...