I'd take (and have taken) the bus between Phnom Penh and Siem Reap in either direction. Paved road all the way. Not much to see along the way, though. Amusing music videos. Assume one stop for toilet/food. No sweat.
I have never taken the fast boat between PP and Siem Reap, only the bus as described. I will cosign that it is not at all arduous, other than a little monotonous. Great value.
Now the bus from Phnom Penh to Battambang and the slow boat up the Sangker River to Siem Reap, that is a whole other story.
http://www.youtube.com/watch?v=zJPPhVKtIt8