Hi Jared,

Go with the 10" FKD10XL, and the real reason is noise. Those CFM numbers are free-air ratings, measured with nothing attached to the fan. Once your silencer, duct, and roof cap add their resistance, the 8" ends up working near the top of its range to move the air, while the 10" barely has to try. A blower that's loafing runs quiet, and 10" duct carries the air at a lower velocity, so you lose that rushing whistle. On the hoods we've put in, that's the difference people notice day to day. It also matches your hood's 10" duct, so there's no reducer adding noise right at the collar.

On the cost worry: makeup air is required once you exhaust more than 400 CFM under most codes, and both of these are well past that, so the 8" won't save you there. Put the 10" on a speed control and you run it low for everyday cooking, with the power in reserve for searing. The 8" will do the job on a 36" range, but every time we've compared the two in a real kitchen, the 10" is the one that stays livable.