Calamity is a 1H'er which means you are going for high CD . The idea with 1H'ers is that you can stack more stats onto two weapons, more than what you can normally. In my ideal Calamity setup, i'd have a 90% CD + OS with a 100% Emerald with an off-hand similar to it. You're looking at over 400% CD with this setup. Not only that, but dual wielding gives you 15% IAS boost so no need for a quiver. Run Archery for the 10% CC.
Manticore on the other hand, is best suited for high weapon damage abilities such as Cluster Arrow, Spike Traps, etc. These abilities are well suited for Manticore because 2H has naturally high weapon damage. The end result is that these abilities will hit for a truck-ton of damage but they aren't sustained damage like 1H is. The playstyle turns into a nuke/burst damage type of play. Revolving around kiting and killing them before they get to you. If you find this more fun, use a Manticore.