I have used both the Thunderhead 100's fixed and the Spitfire Pro 100's mechanical. I found no difference in the two other than the mechanical heads tended to have a better blood trail and the deer traveled less distance after the hit. They both flew the same and I had pass throughs with both and non-pass throughs with both. The ironic thing is the non pass through with the mechanical head went through both shoulders but didn't make it all the way out. The non pass through fixed head went through one side rib and got lodged in the opposite side rib and didn't come out at all. Both were shot at about the same distance 26 & 28yds respectively and both from my Browning Boss Tracker 70lb 33in draw 540grn arrows at about 260fpswith tons of energy!

I have yet to loose a deer with either or yet to damage a blade with either.

To me it's a coin flip. I think you should use what gives you the most confidence when in the field. For me, I will use the Spitfire Pro's only this year.