I wuz t'inkin' aboot these two werds recently.

First of all, what does y'all prefer or use most often?

Second of all, how does y'all use each?

What I was thinking specifically, was of how to refer to each. If you're talking about a new activity, how do the prepositions, ahem, fall?

"I'm starting a new class in the fall"
and
"I'm starting a new class in autumn"

both sound right to me, whereas

"I'm starting a new class in fall"
and
"I'm starting a new class in the autumn"

both grate on my ear. What do y'all think?