I've been a self employed expat for years, living abroad primarily in two different countries - Thailand and Vietnam. So I'm writing this from the perspective of a man who's lived in both countries for years , not days, not a short 1 month trip, years.
Everyone has their place in the world and for whatever reason, living in Vietnam, specifically Ho Chi Minh City Saigon as everyone who lives their calls it , just felt like home. Anyways, I'm going to break down everything I can think of from general day to day living, nightlife, dating, visas, language and so forth.
Thailand really only has Bangkok which is a sprawling metropolis and dwarfs Saigon in terms of size. But Saigon still has that city feel to it, it's just not as huge as Bangkok. Vietnam also has Hanoi in the north and Da Nang in the middle next to the ocean. So there is a bit more variety of places to live in Vietnam. Phuket is a vacation spot filled with Russians and Chinese now, and Chiang Mai has a burning season for 2 months of the year making the air pollution horrible, it's remote and it gets boring after a few months of living there full time.
I personally would describe Saigon is all the best parts of Bangkok and the best parts of a Thai town combined into one place. Saigon is a city like Bangkok, but unlike Bangkok you're still zipping around on a motorbike with some gorgeous Vietnamese girl on the back because Saigon is still small enough where you need a motorbike to get around day to day. You also won't want a bike in Bangkok because driving in that city is stressful, not fun at all, there are a lot of police checkpoints and parking is a headache.
Saigon is still a motorbike city so it's just a fun and convenient way to get around day to day. Thailand Bangkok and Phuket specifically wins this by a mile for better or worse. Bangkok has world class nightlife, clubs, bars and rooftop bars that Saigon simply lacks.
