Paying monthly is slightly more over a two-year period than paying in full, so it's the better deal financially if it works in one's budget.
If you pay in full, at the end of the two-year term, you'll be offered to extend it by paying monthly. You can also request a pro-rated refund at any time during the first two years if you pay in full.
I asked Apple Support recently whether there's an option to convert monthly to one-time payment, but was advised that's not an option.
You're correct that AC+ is not transferable under the monthly option, which makes sense because the term is one month.