I shoot some of both.

Rocket Steelhead 100's (Mech)
Grim Reaper Razortip 100's and Razorcut 100's (Mech)

Rocket Ultimate Steel 100's (Fixed Blade)

These are the four broadheads I really like and have had good success with. All performed very well and were quite durable. I tend to lean toward the Rocket Steelheads for mechanicals because the Grim Reapers do require "some" assembly, but in defense of the GR'sthey are very tough and inflict some serious damage.

I've never really noticed which broadhead(Fixed Blade or Mechanicals) made the deer more dead.

Tune your bow and focus on shot placement. Much more important in my mind than design differences in broadheads.