As bigbulls said. It's all depends on the spine of your arrow. If your underspined a little, then you need to go down to 100, or even 75-85 grains heads. But if your spine is stiff, then you need to go up in weight, like a 125 or higher. Not whether one hits harder or not.

I am sure most of the other posts have put forth enough explanations to warrant using both the 125 or 100 grain heads. In my case I tend to use the 100 grain head because, more often than not, it gives me a better group of broadheads to choose from.

It's not just about total arrow weight. The biggest reason to go heavy is to redistribute more weight to the front of the arrow. Arrows with heavy front ends and lots of drag on the rear, fly straighter through wind and when hitting small twigs. I personally don't give a rat's rear end about any speed gains from a lighter head. Heavier heads fly better when conditions aren't all that good. Quite often when I hunt, there's a breeze or a pretty good chances of hitting a small twig, leaf or weed. I make sure my arrow shaft is stiff enough to handle a higher weight broadhead, which will increase my chance of success in many situations.