Cores never hurt, however neither does some long term planning. If you want to build and use VM's you will need cores, at least two per if not more depending on what you plan to do with them. So if you want, say three VM's, you'll need at least six cores off the bat. You'll always need some for unRAID itself, I would suggest a minimum of two, four would be better. So now we are at 8-10 cores. If you want to run plex, you'll need some more, at least four, maybe more. Better to start with more than you need rather than not enough, although upgrading can fix that. I would say cores are more important, but you don't want to get stuck with a slow CPU either, most in the E5 family are pretty good so I think you are ok.