When it comes to weight, crossbow shooters have the same options and the same variables as compound bow shooters. In general that means 85, 100 or 125 grains. Most hunters are shooting 100s, though the 125 has seen a bit of resurgence in recent years by those seeking more energy.
Without getting too deeply into the theory of general relativity or Einstein's mass/energy equivalence, by going to a heavier broadhead you'll gain kinetic energy but lose speed.
There is, however, another benefit — your bow will get a little quieter with a heavier arrow.
Fixed Pros: Better penetration due to the fact the broadhead doesn't use the arrow's kinetic energy to open the blades.
Fixed Cons: Fixed heads require more tuning to be accurate, and that’s especially true with crossbows. The arrow is shorter; therefore, the broadhead has a better chance of taking over control of arrow flight. That makes tuning essential.
Expandable Pros: Perhaps their biggest advantage is that they tend to shoot with the same precision as field points. Sight-in with field points, shoot a broadhead just to be sure, and you should be ready to hunt. **Most of the time. Be sure to shoot a broadhead BEFORE you shoot at an animal.
Expandable Cons: One of the knocks on expandables is that they can take up to 15 ft./lbs. of energy just to open, leaving less energy for penetration.