My smart***** answer would be: If you have to ask, then you need a HST. Are you plowing fields? Then gear would be fine, and an actual advantage. Are you a surburban homeowner with a CUT? you need a HST.
Upsides, HST is safer to work around people, buildings and on hills, and easier and less stressful for novices to operate.
HST allows precise control, if you need to operate at .88423384938 Mph then that speed is available.
HST allows you to change direction instantly and easily without shifting, it prevents your left leg from falling off. No clutching
Downsides: HST cost more up front to buy, and requires and extra filter to change during regular service intervals
HST makes a whiney sound, especially smaller units. Some whine more than others, some hardly whine at all.
HST produce some extra heat, the larger units have an extra radiator like automotive units with a transmission radiator up front.
HST put less power to the wheels, in other words there is some loss,, see heat production above.
HST are addictive, they are such a joy to operate, you start wishing everything you drive worked like a HST
Why am I so biased in favor of HST? I have owned 5 tractors, 2 gear and 3 HST, My bias is from experience. Unless you need more power to the ground, or cannot afford HST, or just run at 1 speed most of the time, do very little grading or loader work, you need a HST. Obviously I am biased, and an acolyte for HST, just like others here are biased for gear systems. Get a HST, you and your left leg will thank me later
Good luck with your purchase.
James K0UA