I spent nearly every weekend for 10 years gliding, initially at BGA clubs, and then as an ATC instructor. I honestly cannot remember ever hearing any instructor shouting at any student. Taking as read the different aims of the two organisations, the outstanding difference I remember very clearly is in consistency of instruction.
When I initially learned at the London Gliding Club, we students were getting frustrated by being taught different circuit procedures by the various instructors. Although the Club recognised this problem they seemed incapable of sorting it out, at least while I was there. This problem was further demonstrated when I moved to Cranfield and we received a visit from the BGA National Coach in his Capstan. After he had flown several dangerous approaches the CFI was pleased to see the back of him.
With the oversight of the Central Gliding School this was never a problem for the ATC. Yes, the character of each school depended very much on the personalities of of the officers in charge, but in my experience, the quality of instruction is remarkably consistent.