Whatever is lighter because it will give as much volume at a lower cost. Also it may have more surface area for bacteria to grow. Only thing I don't like about rock that is too porous is that detritus will collect in the nooks n' crannies.
I decided to go with mostly marshall island shelf rock in my tank for a clean aquascape.
i agree "Whatever is lighter because it will give as much volume at a lower cost." i like ones with unique shapes n sizes so that it can create a landscape with all type of caves,shelves, taverns,tunnels,cliffs n stuff. as long as theres enough flow into the rocks to avoid dead spots, i would not worry.
they are all nice.. i like to mix. but my main in fiji ultra and some prem. i received some gnarley shapes. i also have tonga and some kailini. there is some super nice rock out there. i just hate shelf i thinnk its ugly.
jackson6745 that is some really nice shelf. your rock is an exception. i am think of the super flat and smooth non porous stuff. blah
If you can afford it, or if you can find someone selling it cheaply, it's nice to actually go to the store and select your LR yourself. Then you can just choose what shapes you want of whatever the store has got.