I don't have the numbers for the GT arrows but if you had an axis 500 cut to 25 inches and fletched with three 4" vanes the 85 grain head would give you about 9.5% foc and the 100 grain head would give you about 11% foc.

IMHO the 100 grain would be the better choice and should produce better flight as well.

I always like to keep my foc percentage above 10% and about 13% is where I prefer it to be.

I know it doesn't sound like much but it can make a differance in the flight of your arrow.