Gutter vacs do suck up water as well and you don't get dirty cleaning the gutters, other benefits are once you get into they are quite quick and you can justify what you charge as your providing specialist equipment, they also attract the attention of the neighbours so is a good marketing tool, I had 2 gutter jobs booked in just before Christmas next to each other so put all the cones out and has my high viz on etc, the amount of interest it generated in the street I was working in was great, I started at 10am and by 1 o clock had taken £280, I bought the Clarke 30 which is all you need for domestics and this set me back £330 but has paid for itself many times over, it's all about being perceived as being professional so would always advise a gutter vac rather than doing by hand..!
Hope this helps.
Graham