The CELTA and Trinity CertTESOL are equally rated, although the CELTA may be slightly more recognisable because it's offered by more providers. That said, schools requiring that level of qualification will always accept either a CELTA or Trinity CertTESOL. I don't know enough about the SIT to advise.
With regards to the schools, the first of the links you've chosen is actually the school where I did my CELTA. I purposely chose that course because at the time it was the only five-week course in Spain, and trust me that extra week is a God-send! I was very happy with the teaching and, for the most part, I found the input sessions interesting. The teachers who were working in the school were friendly (especially the recent graduates from that very course), the tutors were helpful and the school was very well resourced. The biggest problem was the lack of computers. The three (or was it four?) in the teachers' room were pretty poor and were frequently used for teaching, which made the room unusable at certain times. Oh, and the WIFI was a bit temperamental. Nonetheless I got through it, eventually coming out with a Pass A. I would definitely recommend that particular provider (although I'd like to think I'd recommend it wherever I'd done it!).
In terms of employability, you may find the school has some vacancies at the end of the course – that was certainly the case when I did it. However I chose to make my own way, and since doing the course (Feb-Mar 2010), I've spent a summer working in Vietnam and an academic year in Portugal. In two weeks' time I'm off to Poland to spend an academic year there.