What is the best way to get from PP to Sihanoukville?
I am thinking the easiest and most comfortable.

yes, bus from Capitol Guesthouse is the easiest way. We took it in April and it was a 4 hour journey and around $4...just a warning though try and book a seat so that you don't end up on the back row on top of the engine...it gets so hot.
Once you arrive in Sihanoukville, the bus will drop you in downtown...you will have to get a moto to whatever beach you want, the moto drivers are utter b*stards, so don't let them charge you more than $1 per bike to any beach!
There are several bus companies running big a/c buses to Sihanoukville. The buses vary a bit in quality but even the cheapest of the lot is pretty good. The best, IMO, are the Mekong Express buses. $6 I do believe. Usually a bit more comfortable than the rest, toilet on board, snacks and a tour guide pointing out the sites. Phnom Penh Sorya Transport has the most buses running daily. PP Sorya, GST and Capitol all charge around $3.50 or $4. Sihanoukville Bus Schedule

Easiest and the best is definitely bicycle, not sure about comfort (my arse gets a bit sore sometimes). Try the bicycle way, its great!