Hi Oldhammer, there are so many different TEFL courses out there and good research is higly recommended. It's a good idea to look for a course that has some practical teaching experience as well as all the stuff about grammar. I've been recruiting teachers for a long time and can tell you some requirements from the employers' point of view.
Employers will first ask about your teaching practice. So, when choosing a course, the first thing is to ask about how many teaching practice hours they provide. It definitely has to be more than 6, which is what a CELTA course would provide.
An employer would also check your grammar knowledge. English teachers have to prove their competence in it. A very practicle question can be asked in an interview. Besides, understanding how English works will highly benefit your students. Language analysis is included in all TEFL courses but of course it depends on the lengh of the course whether it will provide good knowledge in depth or not. I would try to find a course that would teach not only the theory of grammar but they provide with enough practice to be confident to face your future students. It is important that the approach to English teaching is done without having to translate to the student's language. This is the key when you look for a TEFL course. You shouldn't have to learn Chinese first to teach in China.
If you are thinking in teaching English in Spain there are places as International house, OxbrdidgeTEFL.com, TEFLBarcelona.net that provide classes with practice. I have found this website that explains the difference between them and others: whichtefl.com
Have you thought about what you want to do after the course?