I live in Arizona, so both are reasonable options temperature-wise. You're right though that it would fit the theme better :3
The spring look is slightly cuter and more recognizable, but another benefit of the fall one is that I already have a pretty decent match for the cardigan in my regular wardrobe...
Nope, never have, I do dress up as things for Halloween though, last year I was the generic Inkling from Splatoon, I think this year I'm going to be something from Animal Crossing though, I'm thinking Tom Nook. I think it'd be pretty fun to be him.