In general the difference between fixed blades and mechanicals is flight drag reduction. In essence replacing your practice field tips with a broadhead adds fletching to the front of your arrow. Not only does drag reduce arrow speed but it contributes to flight instability. The mechanical BHs can reduce that “fletch” effect by as much as 50% over fixed blade BHs. That being said getting the arrow to fly straight from the moment it leaves the string is most important and will reduce those types of potential flight problems. A vertical bow needs to be tuned but the guide rail of a Xbow is, in a sense, inherently tuned. The key is to practice with BHs to insure they fly the same as your practice tips, whether they’re launched from a bow or Xbow.
Mechanical broadheads, though saving energy in flight, require energy to open. In general, initial velocities 200Ft/Sec or less dictate the use of fixed blades and velocities of 250Ft/Sec to 300Ft/Sec or more allow for the use of mechanical BHs because there is ample energy down range to open the mechanical blades. In truth, the dynamics of acceleration, initial velocity, drag and flight stability is an impossible nut to crack mathematically or by best guess-timations. The initial velocity, for most vertical bows, falls in the range of 200Ft/Sec to 300Ft/Sec whereas crossbows generally are capable of 300Ft/Sec to 380Ft/Sec. Therefore there is a good reason to consider mechanical BHs for crossbows.
In the end many consider the KISS (Keep It Simple Stupid) approach and stick with fixed blades. Fixed blades are more predictable and there is less that can go wrong. Mechanical blades have to perform properly by not opening prematurely and opening correctly on impact. The tradeoff is mechanical BHs are generally equipped with longer blades, have less drag in flight and fly with more stability. Because there are so many other things that bring luck into the picture I would conclude that you can flip a coin to choose.
For me its 100Gr. Grim Reaper fixed blades for my bow (240FT/Sec.) and 100Gr Schwacker 2” mechanicals for the crossbow (350FT/Sec.). Why Schwackers? They open after penetration. While energy is used to open the blades, no energy is wasted cutting the hide and bones on the nearside. With only three kills experience; all had small holes going in, one had a liver cut in half and two had two ribs taken out on the far side. None went more than 50 yards after hit. Of course most of it was practice and luck and I can’t say a fixed blade would have changed the results.