There are advantages and disadvantages to both. A mech. broadhead needs lots of kinetic energy to use and some designs you should wait for an almost perfect broadside shot. But they can offer better flight with shooting errors and can offer a wider cutting diameter without windplaning.
Fixed heads are always open and can penitrate better, and you don't have to worry about a perfect 90 degree broadside shot, but you are usually limited to 1.5" or smaller cutting diameter and they can windplane easier. also they are less forgiving to shooting error.
I won't go into having your bow tuned perfectly because mech. 'heads are no excuse for a poorly tuned bow.
I have shot both with good success and have a few "pet" heads that I like in both designs but ultimately you will have to weigh the good and the bad and decide which design would work best for you.