First of all, a mechanical IS a broadhead...

depending of good of a shot you are and your setup(poundage, arrow size and so on)

A fixed blade Broadhead will easily penetrate more that a mechanical.. The only thing I can think of where a mechanical is better is in flight.. Your bow wont need to be as well tuned to get a mechanical to fly well... Where as a Fixed blade, you will need to have your bow properly tuned or your broadheads will be all over the place.

I personally use THunderhead 100's and they have performed flawlessy over the years..

just my two cents...