Generally, chemical exfoliants
are better, especially for acne prone skin. This is because they can exfoliate everything much more evenly and efficiently.
Also, if you use manual scrubs that contain shells or anything sharp, it can cut your skin and cause microabrasians that let bacteria in, and lead to more acne!
The only time manual exfoliation is safe is when the product uses polyethelene (plastic) beads or jojoba beads. Proactiv is one that uses the polyethelene, and PTR Botanical Buffing beads use jojoba.
Hope this helps.