Total Brain Melt - Laos North vs. South

We've been asking a few travellers about what things we should do on our 8 week trip in Asia (Thailand, Laos, Cambodia and Vietnam are what we'd like to do). Since talking to poeple we've been persuaded that the North of Vietnam is more special (sorry crap turn of phrase) than the south. We were planning to come into Laos from Thailand at the top and do the Mekong down to Louang Phabang and Vang Viang. I've posted about flights from there to Hanoi at which point I guess we'd head south.

But, we've been told that the south of Laos is truly stunning but I fear with our time restriction of 8 weeks we'll be snarled up with nightmare travelling through the south. Anyone done this and found it to be worthwhile and fairly ok travel wise? From there the particularly vocal traveller we asked said we should pop into the top East of Cambodia (made it sound simple but I'm sceptical) for unmissable eastern Cambodia.

To be honest we'd planned to do Phnom Penh and Siem Reap for Angkor and hadn't considered the top east section. Big mistake????

Should we stick with north Laos?
Should we miss out south Vietnam and maybe fly from Hue down to Phnom Penh.
Any circular routes that people know about that work well?

Apologies for the numerous questions.
TM

I haven't been to Mondulkiri but I have been to Ratanakiri. It's nice but the landscape in northern Laos or even southern Laos along the border with Vietnam is more spectacular. The river dolphins around Kratie are unique though (you can see them in southern Laos as well but sightings seem to be a hit- and- miss thing). You can find gem mines in Bokeo and hill tribes in the north of Laos are more diverse. So, if you've done the hill tribe/trekking thingy in Thailand or Laos there's no need to do it again in Cambodia. I'd suggest flying from Vientiane to Hanoi, making your way south and then crossing over to Phnom Penh. If you then think you still want to see the east of Cambodia go from there.