Mike I've seen a Frontier and don't like it as compared to the WSM. WSM is bigger, heavier built and has better quality hardware, racks, etc. IMHO. Now the new Excalibur is larger than the WSM, has better overall finish than the Frontier (pretty sure it is porcelain coated).
I would definitely give the 20" Excalibur model a hard look instead of the Frontier if I wasn't simply going to buy another WSM.
Also the ProQ WSM Stacker may be enough to ad the capacity, have you checked that out? The WSM stacker has a porcelain finish like the WSM as I understand it, but you should double check.