8" vs. 9" convertor

I am in the process of buying a new TC.

Everywhere I called offered me a 4K stall in a 9.5" convertor. THe last place I called offered me a 4,500 stall in an 8" or a 9".

I'm wondering which would be better, for what, and why it would be better. Same stall, different diameters.

The car is a 69 dart, 602hp @5,900 / 575tq @4,500, 4.10 gear, GVOD. Car see's a few track days a year. Mostly driven 25 miles or less, usually less.