Put it this way...
Eminem is a bit of both. Whether people admit it or not, Em still has credibility in Hip Hop. He drops a single, and it'll be discussed just like Jay, Wayne, Drake, Nas, etc. will. His whole niche when he started out allowed him to tap into the anti- bubblegum Pop audience that hated the likes of boy bands and Pop princesses. Him actually being a good rapper affiliated with Dr. Dre and Snoop allowed him to be tap into the Hip Hop audience. That combination gives you the biggest selling artist of the last decade (of any genre according to Billboard).