I did the GS with GTS. The notes are "comprehensive" and certainly provide a detailed reference library for the future as well, not just to pass the exams.
It's still worth signing up to the BGS on line exam system whoever you train with - you pay the same. Or the Oxford CD. I have a copy of the CD which you are welcome to borrow if you go GTS (it's on loan at present to another student).
The real decision between the three schools is one of style and scale. GTS is seminars (6 people in the room) with the professor. I believe the other two have bigger class sizes.
All a question of choice. They all get results.
h-r