If all else is equal, I'll take the mech head because even out of a well tuned bow that shoots a fixed head true (same as field points) the wind can/will play havok with you. Of course, you can help/cure this with larger fletching and more helical...I PREFER LESS fletching....but that's me.
I would go with mech heads, but don't think they are for everyone.
I have NO problems/worries about penetration, or toughness.
All in all, if you put them THROUGH the lungs...it's gonna die. GETTING them to the lungs (accuracy...even in the wind) is why I PREFER mechs to fixed...if I HAD to take one over the other.
Right now, I'm hunting VERY think woods....a 20 yard shot is a VERY long one, and there is rarely ANY wind that deep in these woods....shooting a fixed (Slick trick) for THESE conditions. Why? Cuase I haven't tried them yet, and the guys I hunt with are "mechaphobic" so to keep them at ease, I'll shoot a fixed.
When in Rome.............