Correct me if I'm wrong, but aren't "Show pieces" particularly large? And if by "top" you mean on the top of the rock pile, not sure how that would work.
Anyway, Fiji is your traditional LR, and comes in a variety of shapes and sizes, tonga is very lightweight and in the form of branches (Not particularly useful for forming primary structures), and IIRC lalo is more bizarrely shaped than Fiji.
As for any differences in things like coraline growth or hitchhikers, that depends mostly on the individual pieces themselves, although tonga (At least my examples) doesn't have all the nooks and crannies of the others, so it's probably less likely to have many extra goodies.
I presume you've cruised some sites with picture samples of their products, so I think it pretty much comes down to personal preference, what do you like?
For our 90g, we went with 90lbs Fiji and 20lbs tonga as an accent, although I suspect we only used around 10lb of it since it's so lightweight (the rest is in a storage bucket under heat and a pump).