While I haven't shot an elk with either head, I've shot whitetails with both. Blood loss with the T3 was massive in all instances for me, and the arrow flies quiet and true clear out to 60 yards, grouping right with my field points. Durability would be a concern of mine, on an animal as big as an elk.
The Montec is a fine head, and is very strong. I can shoot tight groups out to 40 yards or so, but then the groups tend to open up more for me.. I never seem to be able to get the Montec SUPER sharp, and to me, that's important. The Montec also is much louder flying through the air than the T3 is. Blood trails haven't been quite as good for me with the Montec as with the T3, but that could be circumstantial.. I've not lost animals with either head.
Bottom line, if you poke one in through the lungs with either head, they gonna die.