In my opinion it is better to head directly to the beach town (4hrs)and then do a one day trip to Kampot/Kep and visit Bokor Hill. Better still if you have the time spend a night in Kampot at one of the nice guesthouses or Kep for that matter. The trip up the mountain to see the Bokor hill station is over a road that is hard to describe. A 4-wheel drive vehicle is the best way. It take around 3 hrs going/coming.
Kampot is around a 11/2-2hr trip from Sihanoukville and from PP it is 3hrs.
Kep is noted for great seafood dinners (some of the best in Cambodia) and of shore island called Rabbit Island (about 20-40 minutes boat) has some nice beaches to relax on.

What you could do is to go to Kampot directly from PP. and stay over night. On arrival arrange to go up the mountain that day and the next day visit Kep (a short distance away) have a seafood lunch and then head to Sihanoukville. A taxi from here should be around $25 for the whole car. From PP it will be around $40.