who uses sand bags vs. the water based bags? pros/cons?

I think for my self I like water bags for the feeling, and Sand bags more for the roughness and to build my wrist strength up. However when it comes to conditioning the knuckles I enjoy a traditional Makiwara board.

 

All in all I think each one offers somthing different that can be used to enhance your techniques.

If you're talking about the actual striking surface of the bag being filled with water, that will absorb your hits much better than sand. Hitting sand would be like... punching a beach

 

If you're talking about the plastic base of the bag (like a Century Wavemaster) I'd highly suggest filling them with sand. It may be a little more expensive, and it won't be as easy to remove so that the bags can be moved to a different building. However, those bags have a tendency to leak everywhere...it's much, much easier to sweep up a few grains of sand than to mop up some water.

I agree with Monkeygirl. For the Wavemasters, sand it better. I have B.O.B. that has a similar base to the wavemasters and we filled it with sand. I don't want leakage from water.