I only have experience killing elk with two heads, both being fixed blades. Thunderheads are great, they leave mazzive wounds, lots of destruction, and lots of blood. However they fly like crap past about 40 yards. I switched to the montec G5s about a year ago after my dad lost a really nice 330" bull because of a thunderhead. IF YOU SHOOT AN EASTON AXIS OR FMJ WITH A THUNDERHEAD BUY AN ADAPTER RING. We didn't know about it, and the diameter of the shaft is small enough that you can lose your blades on a hard enough impact. The montecs are great heads. I shot a bull this year from 40 yards, complete passthrough enough that I never could find the arrow as it hit a rock and skipped when it came out the other side. The bloodtrail was great and he didn't go more then 40 yards.
People say mixed things about penetration with montecs, but here is my theory with why after watching some tests of arrows being shot in various targets. The heads are solid and they dont give. In several tests I have watched the blades on heads like thunderheads and muzzys sheer off, and the arrow creates a bullet hole but still penetrates a bit. The montecs retain the blades and leave a larger hole, but subsequently the arrow is slowed down more on the contact with very hard targets.
Bottom line is this, ive seen some really bad things happen with mechanicals, including the 2 blade rage that someone mentioned. Had a buddy shoot a rage and when impacted an elks rib it turned about 90 degrees and shot the arrow up through the chest cavity. Worked great that time but he said he would never shoot them again because it could have just as easily turned the other direction. All fixed blade heads of any quality will do the job, if you put them in the right spot. Todays bow have enough power, and all the heads worthy of being mentioned are sharp enough and strong enough to do everything you need if you stick one through both lungs and avoid large bones.