I have used both successfully, and truth be told they both do the trick.
They both have their benefits and drawbacks, and no matter what the marketing spin is on them, they have their drawbacks. But To be honest the best way to figure out what will work best for you is to take the plunge and buy a few different ones and put them through their paces.
I'm constantly trying different heads, even though I have my tried & true favorites. I'm a bit of a broadhead junkie
If you can tune a bow well, there is nothing like a good quality fixed blade in regular hunting distances (sub 40 yrds), and they do work well for longer shots as well, but it takes some string time to figure out how they are effected by wind and distance.
Mechanicals are suburb in windy conditions, very easy to tune (if at all), and they have come very far in terms of reliability.
Any Broadhead, when put where it counts will do the job, some are more durable others more "devastating" but all will work.