Ive been using a mid 70s Raider 12 with 8 speed unidrive manual transmission and find it more than capable for ploughing, I'm just about to set up another C series to do the job with the Kohler 8hp engine and 3 speed unidrive transmission.
This is only my opinion I've never used a hydro box simply because i was warned off them and quite happy with the manual tranny.
At the end of the day its a matter of choice