We are full of many things -- certainly advice !
My opinion is that you should go with the 60" belly mower. You did not say how much area you mow on a regular basis (which is a big factor I think.)
I have a 72" finish mower and 2 other machines with 60" belly mowers, all with roughly the same size and horsepower as you are about to buy. I do not recommend the 72" finish mower for your size tractor unless you have a terrific amount of open, fairly flat area to mow on a regular basis. Other reasons too. That finish mower out there behind you is a VERY unwieldy/cumbersome nuisance much of the time and offers no advantage over the belly mower in any way as far as cut quality. Takes up storage space, involves craning your neck more, etc. My finish mower is a Landpride 72" which is heavy as He double LL and sucks considerably more power to run than the 60" belly mowers -- not just a little , a bunch. So much so that I moved it to a larger utility tractor for a few years and then just gave it a "very long-term loan" to another family member to get rid of it.
Not that you have to get as heavy a finish mower as my Landpride FDR2572 which weighs 718 lbs., half the weight of your tractor, but that is just not a good combo.
I think you are underpowered and light for a 72" finish mower.
Not that you CAN'T but more that YOU DON'T WANT TO.