Thought I would list some of my experiences with TEFL in Korea (Gimhae, Busan) vs Thailand (Pattaya, Bangkok). I taught for a year in Korea before doing two years in Thailand.
Korea - A hagwon (language academy). More of a business than a school. Long, hard working days from 10 am till after 9 pm at night sometimes. Long days, odd hours and tiring but the students are more respectful and interested in learning English than Thai students.
I pitied a lot of my students as many were forced by their parents to do extra studying on top of their already arduous schedule. Schools are more organised than Thailand and follow set curriculums although offer opportunity for games and funtime too, especially with kids. A lot of shady, hagwon horror stories but I got lucky and fared ok.
Thailand- two different government schools with Pratom and Mattayom. More fun teaching than Korea but I felt like more of a rent-a-clown than a teacher. Classes based a lot more around fun and games than proper learning. Students generally ok but largely uninterested in English.
Thai students often lack critical thinking due to terrible rote-style learning from their Thai teachers and are more afraid to speak up for fear of losing face. Some very noisy, chaotic and naughty classes, particularly with younger kids (often leaving the room with a sore throat!).
Schools are unbelievably disorganised and frustrating to deal with last minute changes, compulsory staff meetings in Thai language etc. Much more freedom in the classroom but far worse conditions (40 students to a class, dodgy facilities, some classrooms had no air-con etc). Some very dodgy Thai government schools indeed but I fared OK.
Not recommended for more serious teachers as it will drive you crazy.
Korea - Difficult and expensive initially getting required documents (apostille, etc) and also fairly trapped to the school on an E2 visa. But at least no 90-day check ins, re-entry if you travel abroad and generally better once "in-country".
Thailand - Absolute nightmare. Ever changing rules and requirements for legalisation of documents to get a non-B visa, some of which were so laugh-worthy they were a farce. On top of that, 90-day check ins, re-entry permits to go abroad and having to leave the country and start the whole ordeal again should you change school.
Korea - Sucked balls. Blistering summers, arctic winters, typhoons and not much in between except a couple of weeks in cherry blossom season.
Thailand - Awesome, perfect winters, always warm and usually sunny. Just what the doctor ordered after a tough Korean winter.
Things to do
Korea - Cool mountains and beaches relatively OK but suck for the cold half of the year. Cleaner nature and some interesting things to see. Snowboarding in winter too. Cities cool but nightlife not as fun as Thailand.
Thailand - Awesome, jungle treks, stunning beaches and cool islands. Loved the crazy cheap nights out, pool parties, riding my moto down the palm fringed beaches and forests at dusk. Only downside is the polluted beaches/rivers in parts, especially nearer to Bangkok. A lot of rubbish, poverty and stray dogs everywhere which is pretty depressing.
Korea - Far better, good roads and high speed trains anywhere in the country. Clean, modern cities with lots to do. But drivers sucked
Thailand - A lot worse but more interesting. Standard developing country affair of noisy, dirty and chaotic cities, bad public transport and terrible drivers. More "character" but can definitely become suffocating and after a while you long for a beach/nature getaway.
Korea - I found Koreans to be more contrasted, some very Westernised with good English and interest in the western world, others very unfriendly and curt. More reserved than Thailand and often times ruder too. I considered Koreans to be the "Germans of Asia" if that makes sense?
Thailand - Far friendlier than Koreans but also more likely to try scamming you, particularly in tourist spots. Generally worse at English but more willing to talk to farangs.
Korea - Worst place I experienced in Asia for dating (Korea, China, Philippines, Vietnam, Thailand). Loads of beautiful girls with zero interest in farangs unless they work for Samsung/similar. Humble English teachers particularly looked down upon.
To make it worse, local guys can be the worst. Although there are quite a few girls who are interested in westerners, I only scored a few dates the whole time I was there.
Thailand - A single farang man's goldmine. Lost count of how many dates I scored there. However, pay attention as a lot of them might just be gold-diggers in disguise, particularly in tourist areas. Also, harder to find girls you can have "intelligent conversation" with besides shopping, facebook and Thai TV/music (but also some really cool, worldly minded Thai girls too).
Korea - Sweet teacher packages (free apartment, flights, end of contract bonus and much better salary) but generally less to do outside of work (unless you live in Seoul or Busan). Generally attracts more "serious" teachers (as opposed to backpackers extending their trip).
Weather sucks and locals are less friendly but the country is much more developed, modern and first world. EPIK generally safer than hagwons but you might get placed in the boonies and be the only farang in the village. Good for first timers or those looking to save money as you are totally spoonfed with programs and can put away easily half your salary if you are frugal.
Thailand - Much more laissez-faire and possibly more intimidating to the first time teacher as you aren't babied nearly as much as Korea and with it being a developing country.
More "interesting" mix of expats, some cool, some not so cool. Cheap costs but the salary sucks and it'll be hard to save anything unless you can fully live like a local in a crap, Thai-style apartment, eat Thai food every day and not party or travel, although lets be real, no farang goes to Thailand to live like a monk.
Lifestyle far trumps that of Korea (beaches, travel opportunities, nightlife, climate) and much more of an "exotic adventure".
Either way, both destinations couldn't be more different to each other and both have merits and downsides worth checking out!