I'd go for the security of the 30 year fixed mortgage. Get the biggest, cheapest loan you can now, while money is practically free. You can always play around with accelerated payments later if you want to, but you're locked in with no balloon worries. When interest rates rise, and after you're FIRE'd, you'll have nothing to lose sleep over and you'll feel so smart to have this big ol' cheap loan.
BTW, as far as your lender is concerned, you are planning to move into one half of the duplex, right? Right??
Plus one to lindoro's thoughts on this issue, but I am not above the tiniest bit of smoke and mirrors, provided that you never, ever default on this loan for any reason whatsoever.
P.S. All this approval is conditional based on your ability to afford it and if the property pencils out. If not, fuggeddaboudit.