We use the 18F26K22 / 46K22's because they have two serial ports and are backwards compatible with the 16F876/877 which we used to use. (albeit the analogue/digital configuration has to change)
The 18F2431 / 4431 (with motion feedback module) are used for motor control.

The MFM module tracks the quadrature counts (QC's) for position, and by using a known time interval, velocity. The Microchip velocity measuring module is a waste of space!