Thumb over thumb when firing two handed, but I actually train as much(if not more) firing one handed as I feel it's necessary preparation for likely defense scenarios. In either case, I always use a relatively high degree of grasp pressure and thumbs are tucked for maximum grip strength capability-as if making a fist.
The reasoning behind my grip choices is that since civilian defense encounters so often occur at very close-quarters, I want a strong secure grip for retention and stability purposes rather than one geared toward sport and target shooting. Dexterity and finesse tend to go out the window during a life and death conflict when the fight or flight response kicks in, so I want my chosen technique to be compatible with, if not simulate, natural physiological responses in those conditions as closely as possible. My priority being on prepared to effectively fight(defend) with a gun in probable self-defense situations rather than on pure shooting or precision marksmanship. Also, commonality is a consideration as I train, use and carry both revolvers and autoloaders and want a grip that is equally suitable to both and to which I can essentially reflexively default to. I find the thumbs forward grips less applicable and effective in this context. Just my perspective, YMMV