the idea of using rocks is to support the plant roots, but not to absorb water...
Hygroton is a brand name...they work very well...I use em...cleaning is a bitch-a-roony, but do-able...{baking in oven for a little while works wonders}...
Hygroton is very smooth and lava rock is generally rough and porous...I've heard it said that the rough rock has more to grab onto...I've never had a problem with Hygroton...I also use rockwool cubes to start seeds and clones, I love it also...I dont grow in rockwool only because it holds water too long, but it does fine for clones and seeds...