Would pick Cambodia over Vietnam all day. Vietnam is probably blessed with some better scenery, but the locals are absolute beauts who just see tourists as prime rip-off material, whether you're a flash-the-cash high-end tourist or a backpacker, they'll be looking to fleece you at any opportunity, usually through the use of the dual currency system they utilise. Got dollars? They'll find a crappy FX rate and claim all their prices are in VND. Got Dong? Everything suddenly becomes USD only and yet another crappy FX rate will surface.
Vietnam is also incredibly linear, it's such a long thin country that it takes a good while getting from HCM City - Nha Trang - Hue - Hanoi.
Cambodia has Angkor Wat which trumps pretty much anything Vietnam has to offer on the temple front, the locals are top notch, the country is dripping in ancient and post-war history, plus it's cheap. Really cheap. Phnom Penh is a lovely, Siem Reap also. Avoid Battambang, the Chelmsford of SE Asia.
Only thing that's a bit lacking on in Cambodia is the beaches, Sihanoukville is a big iffy, but you could always get yourself over to Phu Quoc Island (v close to Cambodia but actually a Vietnamese Island). It's lovely.
Borneo, from what I have seen, is effectively an Orang Utan park and loads of forest. Not going to get a lot on the culture side, I would imagine.
