I prefer a fixed blade 4 blade. This is why.
Mechanicals burn lots of KE when they open, not that they don't fly well and kill deer, they do. But with todays quality fixed blade heads flying as well as they do, there is no reason anymore not to. Plus, fixed blade heads last longer in most cases. They are tough as nails and razor, razor sharp.
I choose a four blade because of "inches of slice". Let's say a deer is 10" deep, and if you shoot a 3 blade head and get a pass through you end up with 30" of slice. 40" with a 4 blade. The more you can get the animal to bleed the better, and more humane. I have not found any difference in flight characteristics of 3 blade vs 4, so I choose 4.