That's a very good question. Unfortunately, I'm not going to be much help answering it. My only experience with PWM is dimming LEDs.
What I've read and a little bit I know...
- Fast is good for power regulation. I assume this would be for something like a "switching power supply". A quote from the '85 datasheet: "This high frequency makes the fast PWM mode well suited for power regulation, rectification, and DAC applications. High frequency allows physically small sized external components (coils, capacitors), and therefore reduces total system cost."
- Phase-correct works well for motor control. A quote from the '85 datasheet: "However, due to the symmetric feature of the dual-slope PWM modes, these modes are preferred for motor control applications."
- The maximum phase-correct frequency is half the maximum fast frequency.
- I believe the frequency for fast PWM on a 16 MHz Arduino is 16000000 / (64 * 256) = ~976 Hz. The frequency is half that for 8 MHz Arduinos.
- I believe the frequency for phase-correct PWM on a 16 MHz Arduino is 16000000 / (64 * 510) = ~490 Hz. The frequnecy is half that for 8 MHz Arduinos.
Hopefully, someone else can provide something more informative.
This helped me understand the difference...http://www.societyofrobots.com/member_tutorials/node/230