When you say you are between sizes, is that according to your height, inseam, and Salsa’s sizing guide?

Or do you already know the geometry/fit numbers you’re looking for, and you land between sizes?

If the former, go with what felt better.

it is also worth looking at the stock stem lights. This can give you an indication whether you can go longer on the medium frame or shorter on the large to fine tune fit.

Don’t mess with a set backseat post solely to deal with a frame that’s too short. Seat-post setback should be largely independent of frame size, unless they seat tube angle is different between sizes.