I did my CELTA with International House Prague and was very happy with it (4-week intensive in 2003). I got a job very quickly afterwards too. I can't comment on St Giles because I have no experience with them.
Trinity TESOL is also a very solid, reputable organisation (I used to train teachers in it
so I suspect the choice might come down to the location of jobs you are aiming at. For example, the CELTA is particularly well recognised in Asia and the Trinity in Central and South America I think. Both are strong in Europe. But I wouldn't worry about that too much because a full qualification from either tend to stand hands and feet above many the shorter courses out there at the moment and good schools will know that.
Hope this helps,