Thailand is more known for it. Pattaya is the place most known for sex tourism. Bangkok has 3 main areas, Patpong, Soi Cowboy, and Nana Plaza. Bangkok is a huge city and those areas aren't even 1% of the total land size, so I wouldn't say that Bangkok is really the place, it just has a big reputation outside of Thailand, because most foreigners who go to Thailand land in Bangkok and dont venture down south to Pattaya. But in Pattaya, the entire southern part of the city is bars and brothels. The most famous street for prostitution is soi 6. Another area in Thailand that is more well known for prostitution over Bangkok is Patong, which is in the south, in Phuket. On a scale of 1-10 for how many prostitutes, bars and brothels there are: Bangkok is like a 4, Patong is like a 7 and Pattaya is like a 10.
Cambodia has brothels and bars too, they just arent all in 1 area the way they are in Thailand, more spread out and more low key. Thailand definately has more of it.
HIV rates are very high in Thailand (and I bet Cambodia too), if you know somebody that might be considering going there for that, I would advise against it.
As far as the usual stereotype about these countries and pedophiles:
Thailand doesn't tolerate child sex the way Cambodia does. A lot of people seem to think that there is lots of child sex happening in Thailand and that they just turn their head to it. That may have been the case in the 90s, but most of the pedophiles know that Thailand is cracking down on that with a few high profile arrests within the past 5 years, so many of the pedophiles have gone to Cambodia instead. There is an area in Cambodia called Svay Pak that is well known for that.
Brazil, Mexico, Peru, India. Pretty much all the third world countries have sex tourism, with those ones I named being some of the most popular. Amsterdam is well known for their red light district as well. But Thailand is king when it comes to sex tourism. Its technically illegal in Thailand, but they obviously dont care.