Aluminium is stronger, but more expensive. It's not so well suited to volume production so tends to be less common, and may have a less well developed hull form.
Some companies (eg Ocean Dynamics) only use aluminium, a few more companies normally use GRP but wil also use aluminium on request (eg Zodiac Hurricane, Delta) but most only offer GRP.
I am sure that Novurania had made a rib with an Al hull some 8 - 10 years ago. It was a rather large rib for that period (7 m or so). I think that the Al was sandwiched between coats of GRP in order to reinforce it, but only the GRP was visible at least from the outside, so the looks was perfect.
Michael
