I think a Saigon Do over a weekend would be good. The Sheraton there is a good choice...This would probably be the easiest Do to do between the two. Lots of shopping and eating...nightlife is good too...was better but now a little tamer. I have been twice but didn't get to see much tourist-wise.

Phnom Penh & Seam Reap has the Raffles hotels (which I stayed at last time) which are super! Kinda gets expensive though...