oooooooooooh I can link this with relevance! yay!
Mangle out damages Thunderclap at every single level of mobs hit above 1
When TC hits 1, M must hit 3
When TC hits 2, M must hit 4
When TC hits 3, M must hit 6
When TC hits 4, M must hit 7
When TC hits 5, M must hit 9
It is quite likely at all trash mob levels that M will out perform TC.
It is impossible for M to out perform TC vs elites and VERY unlikely for it to do so against champion packs.
Should also be noted that TC generates ~9.12 spirit/sec and Mangle generates ~8.17 (assuming 1.0 attack speed).
Fun fact: DR:Scattered Blows is more damage, Proc C and spir/sec than CW:Mangle at every level of monsters hit up to 6 (the max number that SB can hit) at which point mangle is far better.
All info I have collected on spirit generators can be found here:http://us.battle.net/d3/en/forum/topic/6607333164
-Druin, the happy monk