they're both the same thing. they're just two different versions.

with the grav bong, you use a bottle with the bottom cut off in a bucket of water, pulling it up while lighting to draw the smoke into the chamber.

with the waterfall, you simply get rid of the bucket. you take a bottle and cut a hole somewhere in the bottom of it, and then you cover the hole, fill the bottle with water, then you put the bowl on, and light it as you release the hole. the water pours out slowly, drawing the smoke in.

i love em both and they both get me really fucked up.

the gravity bong is a lot easier to use and it doesnt make as much of a mess. the waterfall is kind of a pain in tthe ass, but i feel that it works better cuz you can fill the entire bottle with water, instead of the little bit of air left at the top of grav's...

either way, be prepared to get mega stoned, and have a towel ready to clean up your spills