I guess they're even because Artie essentially stole his DTV show. A show which was Nick's brain-child concept and Artie jumped in his grave after Nick generously brought him on board. This after all the addiction issues and instability to give him a regular gig back in September of 2011.
I very much realize Nick's polarizing. But love him or hate him, that show was HIS idea, and he brought Artie in when he could've have chosen anyone he wanted.