For that size I would go with the 12mm thickness.
Well, I've got about 12 1200*600 6mm boards for various surfaces/settings and they work fine for me. But I use them in my home. I don't lug them around much (sometimes from upstairs to downstairs, etc). I don't take them out, and when I needed to, I bought a cloth instead, coz carrying two of those boards, with all my other crap, and fitting them in my compact car isn't straightforward, and I am especially wary of scuffing the corners, which I would guess would be the main wear and tear you would get.
The 12mm boards would be twice as heavy, of course, but I imagine they would cope with more wear and tear.
Some of my 6mm boards have also begun to curve slightly (especially the ones plastered with PVA), but for me that's negligible.
One compromise, I guess, would be to use 6mm but to reinforce them around the edges.