I am most familiar with monohull
construction. For an equivalent strength, aluminium is lighter than a solid fibreglass structure, about the same same as a cored fibreglass structure and heavier than an epoxy
laminate cored structure.
The limiting factor with aluminium is that a skin thickness less than 4mm is not practical. Welding a fair structure with a skin thickness less than this is difficult.
In a monohull
this sort of skin thickness would be used on a 30 foot boat. For a catamaran
the length is somewhere around 40 feet.
So below 40 feet an aluminium catamaran
is likely to be heavier. Above 40feet the aluminium catamaran will be lighter than a solid structure below the waterline and cored above. Above 50 feet it is likely the aluminium will be slightly lighter even compared with a cored structure.
carbon/Kevlar/E-Glass cored structure will be lighter again.
A cored structure is more prone to damage (delamination and water
penetration) and this must be weighed against the strength and cost saving advantages.
In short a monohull of less than 35 feet, or a catamaran less than about 40 feet is not sensible to construct from aluminium, unless there are specific requirements.