If you have a properly functioning block heater, that has been plugged in for at least 4-6 Hrs, you will NOT need a crankcase heater.
The block heater heats the coolant, which in-turn heats the block. The heated block than heats the oil, and the rest of the internal engine components.
Its much more important to use the right winter oil than a cranckase heater. I use Mobil 1 SuperSyn 0W40. This specific oil, and grade, is recommended and approved by Mercedes for our Diesel cars.
See:
http://www.whnet.com/4x4/oil.html
Phil