The 1/2" is a different design than the 3/8" ( pilot port vs. direct acting) allowing it to operate at higher pressure. It will still freeze the diaphram shut in cold weather, if you have any moisture in your lines.
I have 1/2" SMCs. Lately i have had a problem with them freezing, I cant figure out why though. I have water traps and drain the tanks, before anyone goes crazy on me. I like how fast they are compared to the 3/8" though.
why not 1/2 dont seem to freeze up as easy and the quicker it goes down the quicker it seems side to side and all that good stuff.... also I think I'd rather be faster and put in reducers than slower and be stuck with it like that!