I'd recommend MB's class since it's the only I've taken. It was a 4 hours drive, and it's right next door to Manhattan so you can visit NYC while you are there. I was not fortunate enough to meet MB, wish I did, Lora(his wife) is very knowledgeable too, the class was thought by Stu Rosen, who owns MB now, and is very helpful before, during, and after the course. You can call him up on his cell, and he would be on a job and still talk to you and help you out. He won't try to sell you anything either. I can't say much about TOG's class or NSRA. Personally I'd stick to DG(dirtygrout) for stone help, most of the guys above are members there including myself.
My main issue with stone restoration is that in my area after doing some research was that the top companies were charging super low rates, imo it just didn't seem doable for me, last year they were grinding @ $2-$3, didn't understand why so low. Had several estimates/bids for jobs last year but clients said we were too high, we're at $3 and up. To me it's not worth going thru grits, man labor, for $2 a foot.
