RE: 100 grain vs 125 grain
Mahly13 is on the right track with FOC. When shooting fixed broadheads, I find this variable in the flight of an arrow a little more important thatn speed or KE during hunting season. This year I had to switch to a 125 grain broadhead because my FOC was only 7.5...Just by rasing 25 grains I moved to 9.8. My groups have tightened up with broadheads and I couldn' t be happier. Actually the increase in front point weight has also increased my arrow momentum...which means it actually penetrates better. More drag is now required to stop the forward motion of my arrow.
But it could go the other way, you may need to drop down the change your FOC in your favor. I do think there are more broadhead selections in the 100 grain area, but many companies have been coming out with 125 grain version this year from last years models...Makes you wonder a little doesn' t it.