The most important deciding factor overall is the instructors.
I tried gliding many moons ago. Different (amateur) instructor every week so there was little continuity from week to week. Also these guys, although nice chaps, were not always very proficient instructors. It was not something done as a living. but rather something done now and again occassionally.
The value for money is pro-rata to the ability of the instructors.
I started PPL training last year and on the basis of the above looked for a school (using Flyer magazine's website) for a school with a few full-time instructors and took a trail flight with one of them.
I've been happy with my choice of school ever since.