Tractor PTO is mechanical/gear all the way through.
Engine power transfer to the PTO will be the same whether tractor has a gear transmission or an HST transmission as PTO does not interface with the wheel transmission.
Engine power transferred to tractor wheels will be higher with a gear transmission.
HST is easier to use and speed choice infinite but it is pump/hydraulically driven, which is less efficient transferring energy than mechanical/gear drive.
Only a few tractor brands offer an HST transmission on tractors generating more than 60 engine horsepower. Over 60 engine horsepower most new tractors are available only with gear + shuttle shift.
HST and Gear/Shuttle are equally reliable.
HST and Gear/Shuttle are more reliable than traditional Gear/Clutch .
VIDEO:
Neal Messick, Messick's Tractor, ( five stores in Pennsylvania )
relates that Messick's sales are 90% HST in tractors of 60 horsepower or less.