Vietnam

Look at temperatures as you might want to move your dates forward. We went to Vietnam, Cambodia and Thailand in early October and it was very hot.
Vietnam: Hanoi , Halong Bay. Stay overnight on a small junk in Halong Bay.
Saigon may be listed as Ho Chi Minh City but once you’re there, you’ll hear it referred to as Saigon., a lot to see in Saigon. Also Mekong Delta. Visit Hoi An and Hue too. Fabulous food.
We really enjoyed our time in Vietnam and would like to return.
Cambodia tore at our heartstrings. So much need there, tragic history. Enjoyed the capital, Phenom Penh, wonderful food.
In Thailand, we only visited Bangkok, are glad we saw it but don’t have a desire to return. Loved our hotel, Shangri La, right on the river. Fabulous service.

I’m thinking of going around Thanksgiving/early December. It’s actually historically lower average temperatures in November/December for both countries as they move into the winter months. But it’s going to be warm weather regardless given those climates. I live in Atlanta, so I can probably handle it. Appreciate the advice and tips.

If Hanoi, go to Halong Bay. You should look to stay on the boat at least two nights. Otherwise it is out and back too soon.. Halong Bay is about 4 hours from Hanoi. So that is 4 hours there, one night on boat and then back the next day. We had some people on our boat that did only one night and they were taken back in by separate boat and missed kayaking and the caves and, fortunately, did not even know what they missed.
If you are from Atlanta you can easily handle Vietnam weather.
Get a guidebook on Vietnam. It would be easy to spend you whole time there with no Thailand. So don’t shortchange Vietnam or Thailand by putting too much together.

With two weeks, I'd pick either Vietnam or Thailand.

We liked Hoi An in Viet Nam in addition to Hanoi and Ho Chi Minh City. We flew into Hanoi, stayed for a few nights and then flew to Danang and took hired a driver recommended by our hotel for the trip to Hoi An. For our 10-day trip in Vietnam, we divided time between those 3 cities (Hanoi, Hoi An, and Ho Chi Minh City) - there are day trips from each and a beach near Hoi An.

For Thailand, we really like Chiang Mai and have spent a month there in the January/February timeframe - we've made 4 trips there in the last few years. Chiang Rai is an interesting smaller city and there's also lots to see in Bangkok - it's an exciting city! We're not so keen on the beaches in Thailand but know that many go there for that reason.

If you decide to spend time in both countries, check Air Asia for one-way flights between them.