I'd probably lean towards the flat bottom bin with lots of fan power on it but we only dry yellow corn, no specialties.

Since you are looking at a hopper bin that is ~ 45' tall, I'd recommend going with a little more height on the 24' bin.    The added cost will be minimal in the grand scheme of things and when you go to a bigger combine you'll appreciate the extra bu.

It won't take very long for a 9600 with an 8 row head to be sitting with the 8000 bu hopper bin full.

Going 45' tall with a 24' flat bottom bin with a floor in it gets you a 14,000 bu bin, which would get you about 11,500 that will run out to feed the dryer (using your numbers for the cone).

Its fun spending money for other people   :-)