I have a manual, I like it. Does what I need it to do & there is nothing that can go wrong with it other than a cable. With the other you'll have pumps, switches, wiring, hydraulic lines, gizmos & doo-hickeys that can foul up on you. Buying one new & you won't have those worries for a very long while though I'd imagine.
Biggest complaints I have seen is they don't move fast enough or its hard to tell where they are at trimwise unless you get an indicator... Biggest complaints I have seen regarding manuals is the cable can have or get slop in it & not travel up & down as well as it should, and the fact that you lose a little floorspace with the controller there.
Think most of it will boil down to personal preferance, I've only known one guy with a hydraulic diverter & I never heard him make any comments about it whatsoever. He disapppeared awhile back though, so can't get his input....
One other thing I've heard that you don't get as much up-travel as you do with a manual diverter. Not sure if that is actually true, just something I picked up along the way. May be worth asking about though when you call to get one?