A "metal hardtop" refers to the solid roof of a vehicle that is made of metal instead of a fabric or convertible material. It is a fixed roof that does not retract or fold like a convertible top. Full definition
But the SLK offers something the other two don't: a fully retractable metal hardtop. (articles.orlandosentinel.com)
The FT - 86 Open features a folding fabric roof as opposed to a heavier folding metal hardtop, and just like the coupe, will have a 2 +2 cabin. (evo.co.uk)
The 335i convertible starts at $ 50,400, and it comes with a retractable metal hardtop, as opposed to the 135i's fabric roof. (automobilemag.com)