One of the biggest decisions you will make as as an English teacher abroad is where to teach. Aside from your personal preference for cities or smaller places, there are several factors more uniquely relevant to teaching and/or living as an expat.
There are generally just more jobs in cities. First of all, this means you have more choices; when you search for a job, you will find thirty language schools instead of two. Also, it may keep employers a little more in line because they know that if they don’t live up to their end of the deal, it isn’t that difficult for you to move to another school.
There are also generally more native speakers like yourself in cities, which means the TEFL market might be more competitive and lower paid than elsewhere (despite the elevated cost of living in a larger city) simply because of the great supply of native speaker teachers. Socially though, most English teachers see this as a good thing — it is possible to have an active social life with people who share your mother tongue. Some counter that in a smaller town, locals are less likely to be jaded towards foreigners and more likely to show them hospitality. In my experience, this depends very much on the location, so much so that it is almost impossible to generalize. Nowadays in many, many places, native English speakers, while in demand as teachers, are no longer a novelty. You will not be the first nor the last native English speaker to pass through a school, and it is unlikely that you will get special attention just for that. In a smaller town, people used to having a lot of other people around may feel lonely.
Being in a big city may also mean greater accessability to cultural event — concerts, museums, nightlife — and general merchandise (there will be a Tesco or something similar). You may be able to travel more on weekends because your city is a transportation hub. In some smaller towns, things that you take for granted may just not be available. While in bigger places, you are probably less likely to feel the need to seek out English language reading material, it is likely to be much more readily available than in smaller places. Similarly with high speed (or any) Internet.
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All that said, you will likely have a more unique experience in a smaller place. In many big cities, while they may be packed with cultural events and restaurants, you will feel like you could be anywhere in the world. You will likely already be socializing with other English-speaking people and will definitely be working in English. You may also encounter tourists frequently. In a smaller town, you are more likely to be exposed to local culture, if only because of a lack of other options.
Whether or not you can save money depends on a whole host of factors, but there are reasons to believe your chances are at least a little better in a smaller town.