If you are going to live like a typical teacher, living in the cheapest accommodation you can find, I think you will find Cambodia cheaper to live than Thailand. Both are cheap, for sure, compared to the west, but you will probably be able to rent a place for about a tenth of your salary in Phnom Penh as opposed to Bangkok, for example. I know some of the teachers where I was working were renting for under $100 a month and now there is an oversupply of accommodation in Phnom Penh so the prices should remain low. Meals are about $5 a person, even in the finest restaurants and of course street food is so cheap they practically pay you t eat it.
There is probably more to see and do in Thailand then in your typical Cambodian city or town where you may end up. The infrastructure in Cambodia is not great, so to get to the beach, for example, if you live in Phnom Penh it will take you 4 or 5 hours even though it is only 221 kilometers since there is only a two lane road.
In Thailand, there are actually highways so you can move a lot faster from place to place. For example, if you want to go to the beach from Bangkok it is only a matter of an hour or two, depending on where you want to go. Going by bus is equally cheap in both places with virtually any bus ticket to anywhere for $10 or less.
Tourism is a business and Thailand does it best. If you want to see "things" like Temples, beaches, museums, interesting shopping and so on, Thailand beats Cambodia hands down, but if you want to see "natural wonders" then you have a fight on your hands which country offers more.
Other than Siem Reap, which is out of this world for interest, places like Phnom Penh offer very little. All of the tuk tuk's carry a list of tourist attractions and there are about five. You could take a year to see all of the attractions in Bangkok, but it is, of course, a much larger city so perhaps the comparison is unfair but if you compare a small city in both countries, you will have more 'formal' things to see and do in Thailand so it just depends what you are looking for, like anything else, I suppose.
There are so many international schools in Phnom Penh it will be very easy for you to find a job if you are qualified. I was only associated with one school so I am certainly not an expert, but if you look at some of the job sites I referenced in many blogs , you will find many jobs and if you happen to be a little older, Cambodia welcomes you because the population is so young.
Thailand also has many international schools where you can work and the pay is higher, but it will probably cost you more to live. If you are idealistic, stick to Cambodia since it is such an emerging country you will be doing a lot of good working there.
If you would like to add anything else, please do so. What have I missed or what would you like to know more about? Again thanks for the letters of support I receive every week. I am glad this blog is helpful t you.