Shred and Mangle has the same weapon scaling but Shred has 20% extra damage on bleeding targets. Mangle costs 35 energy whereas Shred costs 40 energy. If you use shred you benefit with 5% more damage per energy spend. If you use mangle you win with 12.5% more energy.
Shred vs Mangle: Mangle dose win with a very small number, The benefit for using Mangle is that you'll build the combo points faster and the rotation will feel slightly (If any difference at all) smoother. Thought the rule will be...
* If you have 70+ Enegry Shred
*If you have OoC up and thrash is up Shred it(If you where to use mangle)
*If you have berserk up use Shred