Actually the damage from one tic of MS and the initial damage from SWP are pretty much the same, and spamming SWP makes DI proc faster. I am not sure MS would end up being higher then spam SWP, if you picked DI but if you have ToF MS would be ahead. But seeing the buff to MB, i think DI will be a clear winner in single target situations.
Also MS is not op atm, it is in holy spec, it is good in SP but not op.