Optiminer should be faster. Do not forget that you have to subtract 2.5% for the dev fee on Claymore while optiminer shows hash rates with dev fee subtracted already!
That's true, but Claymore has fan control! /sarcasm
It depends, try both yourself. For example, OM 1.6.2 is faster than CM 12 on my stock 290, but slower on my 290 that is flashed to a 390.