they perform exactly the same.
I like 1mil much much better because it's softer as in not as stiff so it conforms to the body better etc. 2mil use to be all the rage because there was this theory that it lasted longer. That's been disproven so 1mil is becomming more popular since it's softer.
Really, in the solids, either one is just fine. It's just a preference thing. But, if you get prints, I'll warn you that a 2mil woven print is like cardboard *L*. (btw.. they will soften a little after washing, but not much) Even 1mil woven prints aren't the softest thing in the world, but a 2mil is crazy stiff and thick. So if you get a print, really shoot for a 1mil and even better, get a knit print and not a woven if you can avoid it.
That's just MY take on it. Some people love sewing with the 2mil.. prints and all.. I'm just not one of them. If you plan to make a large purchase, get samples first so you can feel it before you buy.