I believe that the C series and smooth can series are the same motor. The C series of motors are nicer in that the integrated cooling fins do help to keep the motor cooler, but the C series has its own problems, such as the end bells falling off.

The XL motor (any of them) can and will display brutish power and are nice motors for the money; but are inefficient. This means that they will run hot. I run a 9XL with a homemade heatsink that completely encapsulates the motor and I can run on a track for 20-30 minutes before the motor begins teetering on the 200*F mark and I have to let the motor cool.