ORIGINAL: bigbulls
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.
I have used Stingers since they came out and did alot of testing with them as I am a sales person for them so I wont sell anything that isnt up to my standards! I agree with bigbulls above. Anf the 100 grain will give you a bigger cutting area (surface) and larger holes. 15 grains shouldnt affect the FOC that much. I am not in agreement with people who shoot light poundage should shoot a lightweight BH, relatively speaking. As long as the arrow is matched to the BH and the bow, go as heavy as you can as heavier BH's have more advantages (usually). The 85 grain is a great BH and anyone with wind problems for shooting long distances, this BH really helps.