Thanks for the replies. Yes, I think Cambodia this time (and maybe Hanoi next trip).
Given that we only have 8 days and want to see both PP and SR, I'm trying to figure out the domestic travel bit before choosing flight from BKK.
With time so tight, think we'll pass up the taxi or boat trip between the two cities and then have to do it again for the flight out.
I think we'll fly BKK-PP , domestic flight PP-SR and then fly back to PP again a few days later. That's the best I can come up with.

Apart from domestic flights being more expensive than return trip by taxi or boat, is there anything wrong with this plan?