Amumu skill order

So I was doing math on w vs e and if I did the numbers right w is more dmg per second against the big monster in the camp than e, and it doesn't take much hp on an enemy to be true in ganks. For the most part people take the e max over w max route though. Is the dmg reduction and increased burst for ganks worth it on a champ that wants to power farm to 6 and can do great with the farm for a damage heavy tank build, or did I math wrong?