Expert advice is usually to use 100 grain broadheads with lightweight carbon and the lighter aluminum shafts, 125 grain with heavier aluminum ones. That said, the draw of your bow and personal preference are also important. The more powerful your bow, the less impact weight has on distance (we're only talking a difference of 0.057 ounce here). Additionally, more weight gives more momentum and potentially a cleaner kill. However, you shouldn’t try to put a 100 grain head on a heavier shaft. There's then too little mass at the front of the arrow, and flight can be erratic as a result.