I know there are a lot of different grains for broadheads and field tips. I know some people who prefer 85 grain, and some who prefer 100. What difference does the grain make?

Weight (or grains) of your broadhead (field tip/point etc) is all part of the equation for/to your FOC (Front of center), spine of your arrow, weight of your arrow etc.

100 grain is the most popular choice. Higher or lower number will affect the issues that Rob mentioned. Unless you are trying for a very specific set-up (like a high foc or ultra-light arrow), then 100 grain will work nicely with your XT hunters. 100 grain also offers the greatest selection of broadheads.