The self-propelling is done by use of the tines digging and pulling the machine along. Because the motor is usually on top of the tiller tine area it is subject to a lot more dust and dirt ingestion. Make sure to service the motor and air filter more often. Rear tine tillers are the most comfortable to operate, but usually cost more because they have so many more parts needed for the propelling, and to separate the tiller operations, from the drive system.
Several companies offer counter rotating rear tine tillers.
These are used to break down the soil as the rototiller is driving into it. We have seen in our shop that these units wear tires out sooner, as well as putting an unneeded amount of wear on gears, and shafts. Look instead for a faster more controlled tine speed in a standard rotation. Geared seems preferable rather than chain or belt systems which may tend to slip or wear. Many times the gearbox assembly cost makes the repairs exceed the replacement cost of the complete unit.