Just avoid the "automatic" transmissions. They are junk, and a PITA to operate. Just like a vehicle, you have to shift into forward/reverse to change directions. Hydro is the ONLY way to go, especially for the little more money it will cost over an "automatic".
The old gear transmissions work fine if you have a nice, flat lawn with NOTHING to mow around. In that case, deck with is the only consideration you really need. Engine size is a consideration if you have thick, luscious grass. That said, I do like my 22hp Briggs in my D130.
I went with a JD130 because its easy to bypass the reverse implement switch and the layout was comfortable to operate (bad back). Most modern mowers don't let you go in reverse with the blades on without some convoluted array of buttons and such, some just kill. The JD was a simple jumper wire to fix that. I have lots of shit to go around, I spend half my day of mowing in reverse...