Hi Barry,
Either is fine - whatever works....
Since you're only looking for velocity and not position (right?) you could either use the motion feedback module in Velocity Mode (something I've never done myself) or use a standard timer/counter as outlined earlier.

As for the timer interrupt... A neat feature of the PCPWM module is that you can use IT (or rather its timer) to generate an interrupt every Nth PWM cycle. That's what the PostScale setting is for. But the available post scale ratios are limited so if your PWM frequency is high and you're looking for a low(ish) interrupt frequency it might not be the best to use.

Again, either works.

What's the lowest speed you expect to run the motor at?

/Henrik.