The 18F2431 has a specialiced PWM-modul for motor-control. 3 PWMs can control 6 Output-Pins for high- and low-side.
The PWM can produce normal PWMs or center-justified-PWMs. The frequency of the device can be very high to produce fine PWMs...
The 18F2431 has very fast ADC !

The 18F-devices have a better RAM-organisation.