I know this is an apples & oranges comparison...until you see about
tiller rotor speed/ground speed.
I used to have a
<font color="teal">Yamaha TerraPro</font>.
I had the good fortune to win a radio sweepstakes that included: Yamaha TerraPro YFU350U ATV, 38" tiller, hydraulic system, 50gal sprayer w/16' boom, 100gal sprayer w/24' boom, 42" woods bush hog, 48" woods finish mower. This was top shelf stuff, but in the long run, it's a gimmick. Mostly due to the
proprietary 2 1/2 /forums/images/graemlins/confused.gif point hitch. (I humorously called it that because the mowers were semi-mounted, yet the tiller was 3 point mounted.) Of course, there's no future to anything proprietary (look at IH's Quick Hitch, and AC's Snap Coupler),
in my opinion.
The TerraPro had a 2000rpm rear PTO rated at around 11HP. 1st gear of the TerraPro was 1.5mph.
The RT38 woods tiller (4 tines per rotor flange) ended up with about 210 rpm at the rotor.
My soil is a sandy loam with very little clay (when we had it tested at the lab about 20 years ago, it was someting like 30%sand/45%loam/25%clay). The first time I tilled it, it scratched about 3.5" deep. The second time I tilled it cross-ways from the first, about a week after the first time and it tilled it pretty uniformly 6" deep. I tilled it after growing a vegetable garden, and it was at least 8" deep, this was the first time the TerraPro had trouble tilling from traveling too fast.
Gear/hydro - this debate will go on forever. In a
laboratory setting hydro may prove to be advantageous. But, in the real-world, gear drive will get the job done as well, given time to do the job properly. If you feel like your gearing may be too tall, you can always opt for the 6 tine per flange tillers, they will chop 50% more per revolution.
That's my $0.02. and your mileage may vary.