The JD is the best tractor, but the NH is the best deal. You'll have to make the decision of which is most important to you.
Any tractor with an HST transmission can have transmission stall. It actually happens quite frequently in the higher gear ranges. On a gear tractor, if you pull too hard in a high gear, the engine stalls and you have to restart the engine. On an HST, the transmission stalls and you just shift to a lower range or go slower. It's like your FEL. If it can't lift something, the hydraulic pressure relief valve opens and the FEL stops without killing the engine. On the New Holland HST, the pressure builds inside the HST transmission up to a maximum of 5000 psi before the relief valve lifts. If you are in a lower range and have the engine rpm not high enough, you can stall the engine, but this doesn't happen that often with HST.
If you are mowing with a tractor, the cutter is attached to the PTO. You can absolutely stall the engine if you get into heavy grass and you are going to fast. You won't stall the transmission though. The PTO is all gear driven and has nothing to do with the HST transmission. If you get into heavy grass and the engine bogs down, just slow down a bit and everything will take care of itself. That's the beauty of HST and its infinitely variable speed. If you are pulling a plow, the transmission can stall. This is why plowing larger acreage is best done with a gear tractor instead of an HST. The HST shines in applications like FEL work and mowing where you have to vary your speed and reverse direction constantly.