The 8ft AL (aluminum) model is 78lbs, while the 8ft VS (fiberglass) is 125lbs. However, when you get up to the 12ft model we prefer (we have a lot of guests to shuttle to shore), they are both approx 200lbs. Odd.
If you are referring to the lightweight versions that only have a small raised board that keeps your feet out of any water collected in the v-hull, they are all notably less heavy in aluminum. The great advantage of these lightweight models is there is more distance between sitting on top of the tube and the floor, since there is essentially no mid-floor. However, we really think we need the little storage box on the heavier model, for lights, sweatshirt, whatever. May be poor thinking.
Again, I'm really the odd ball here, as I prefer the non rib inflatables.
One of the best features with a true inflatable is that it can easily be deflated and stowed below even at sea (in the salon if necessary) should one encounter a really bad storm. It also stows on the foredeck right side up resting on a couple of fenders, with the motor on it (those of us with larger boats) without damaging the big boat and as a lifeboat in conjunction with a liferaft.
I also like the huge interior (ours is a 10'6" with alloy floors) which can easily accommodate two 55 gallon drums (of fuel or water), 6 people and their gear or a whole lot of provisions for a voyage.
The dink runs at close to 30 knots w/2 of us, but has planed out w/4 and some gear, with a 15 hp, 4 stroke. Total weight of boat and motor is roughly 235#.
I'm not trying to sell you on this, just offering another thought you might not have considered. The ribs are definitely superior for davits as we've had to reglue the lifting points forward once, but that's a minor inconvenience for the advantages of the true inflatable, in my mind.
As mentioned above though, I don't know if we'll be able to afford another Mark !, or if they are even going to be manufactured when the time comes.