Many seem to favor water pumping nowadays. In my experience water pumping is "safer", but of course going too long or too hard can cause water retention.
But overall, water retention is usually much more prominent in air pumping! (In my experience as well, I do have several pumps.)
Of course air pumping is a bit less messier, as there's no water involved. So it can be done in front of your TV, computer etc., much easier. But then again, one could have a bucket for water and still do hydro pumping in front of the computer, TV etc..
I favor water pumping, I do my sessions in a shower cabinet. The heat doesn't escape that much, so I don't let the water run all the time. Only when I'm washing and rinsing myself.
I find it very relaxing, where as air pumping I've always felt more unease. Also with air pumping I've had far more blood blisters and water retention.
I think both ways have their time and place, but I do see myself as a water pumping guy.