I for one did a full resto on my ole 40 tub with steel, and two years later thanks to the undercoating I used (rhino liner) the bitch is rusting again thanks to the cracks in the box liner I used....so, if your comparing weights as a decision to which is supior...I...the once "steel is stock and that's the best", am leaning to the composites...I do know that when we weighed vehicles between my 40 all steel and my buddies 40 all fibre, they came in the same. Fibre glass needs a tremendous amount of material to duplicate stength and be sure to use polyurethane body bushings to alot for flex or else suffer paint cracks due to flex. Aluminum is intence to repair and paint, fibre glass is also mind numbing to repair when it comes to body damage, steel is user friendly, let these be the options which choose your rig's spec's. If I was building a competition rig, I would run an Aluminum tub un painted with a nice clear coat. Dents may be a problem, but aluminum is beutiful in the raw....even big fat welds. As a mountain biker, the fatter the better.
When I swapped tubs, I felt the aluminum tub with aluminum cowl to be around 2/3rds the weight of the steel one, plus or minus a little. Keep in mind the 175# weight rating on the Aqualu tub is for the tub only, not the tub and firewall. The malotte fiberglass tubs are almost as light as the aluminum ones, but the gozzard tubs are as heavy as the steel tubs, due to the strength built into them.