Laos a poor country just means it's cheaper for you as a rich westerner. It also doesn't mean that their quality of life is any worse. If the money you spend is distributed evenly then you are helping sustain or improve their own little ecconomy.
If I had to choose between Laos and sCambodia... well Laos is the clear winner.
But I also like Cambodia, though I do get a bit sick of seeing it over run with pedophiles.
Unfortunately the places like Siem Reap are also the places that attract a lot of tourists and a lot of scammers. Of the beaten track you'll find a lot more interesting sites and people.
They're also communist countries as is Vietnam and China. Not that that bothers us much these days.
Both countries are close. If you have time, do both !