I think the main advantage of an on-site course is that you get actual teaching practice as well as feedback on your teaching from teacher trainers and classmates. I have a CELTA, and while I don't believe there is a big difference between many of the different on-site courses, the CELTA is very widely known and internationally recognized (and actually monitored by Cambridge to ensure courses in different locations meet their standards). But, if you are only thinking of teaching in one country (and only for a year and only as a back up plan at that) a course at a local school that is well-regarded might do just fine.
If you are just thinking of tutoring individual students on your own, being able to tell them you have a certificate might help a bit, but it's certainly not a requirement (though it may be for a work permit in Chile but you're not likely to get that easily anyway as a self-employed tutor). I don't know about Chile, but I have heard that in some places in Asia - generally where the demand for teachers is great - employers don't require a certificate or may not really know the difference between them.
The other reason for doing a course though is just that the communicative, task-based methods that many modern language schools use or claim to use nowadays are in fact pretty different from traditional language learning methods in the US, at least in my experience. You need to get students to talk and often are supposed to guide them through questions about the grammar so you aren't just lecturing on it. So a course can at least point you in the right direction - while a four-week course can hardly be said to be throrough preparation for teaching, I can't imagine teaching classes, alone, without it.
Guy Courchesne is a teacher and teacher trainer in Mexico, and may have further advice. TedKarma (who is also a Bootsnall member)is based in Thailand but has some good info on TEFL training at
TEFLDaddy.
Dave's ESL Cafe is a great resource for EFL teachers and my own writing on TEFL is
here in the form of a bootsnall logue.