the disaster weapon would be better, but it's not worth to get them to lvl 20 skill anyway as they'll be replaced for better SR or SSR of the same element.
having other element weapon in your team will do nothing but benefit the overall atk, but no real help in a battle as you need the elemental bonus from the weapons...

that SR disaster weapon will give you an extra 13% assault to your grid (at lvl 20 skill), whereas the SSR will give nothing if it's the wrong element.