The advantage to mechanicals is that they are suppose to fly like field points. I believe if you properly tune your bow that your fixed broadheads will fly like your field points too.
I like to keep things simple so I prefer the fixed broadheads. Keep in mind that I shoot fingers because I want to eliminate the need for a release and the possibility of its failure or that I forget it.
Could I shoot better with a release - probably yes, but not enough to worry about for hunting.
Would mechanicals fly better than the fixed broadheads - probably, but not enough to worry about for hunting.
Is there anything wrong with using a release or mechanicals - absolutely not. To each their own!!!