A twelve has a boatlod more tension on it and either its built like a tank or you tune down. More to the point though is sound. A twelve has two things going for it....that neat jangly kinda autoharp sound and a booming bass. A lot of twelves are overbuilt and supposedly can take it. Bullshit. Unless you're using relly extra light strings, over time its going to wear down on it.
But the real point is that the sound is so much better (even on one of Kendall's hated "overbuilts"---he has an Appollonia as I recall?---nice ax) when you tune down. Then if you go with heavier gauge strings, you'll find the bass gets bigger too! Rick said that he'd found out that Seeger was using a LaBella .068 Silk and Steel on the 11th.
My advice, for the absolute nothing its worth, is to try a set of mediums (.012-.052) and tune down one and a half steps and see what you think. If you got some real guts and real fingers, try the GHS Leo Kottke set, a heavy gauge (.013-.056) and try tuning down either one and a half or two. Bet you're amazed at the sound.