On an archtop, I've always found flatwounds superior, particularly Thomastik-Infeld. I tried heavier than the .10s I always use on electric and Gypsy jazz guitars, and found I got all the extra fatness I needed from .11s. My fingers, anyway, can't do .12s or .13s.
I say try a bunch of things, and find your own preferences. I often string mine with Argentine GJ strings as an alternative when I want less smooth and more dynamic.
I like flatwounds on my archtops. Currently playing Rotosound rs200 12-52 .... get a lot of compliments on the sound and they are only $15
I use LaBella black flats lovely
Stupid question time: Are you using flats on purely acoustic archtops or ones that have a floating / neck pickup?