I think they're both equally good names but since you bring up numbers..
KingOfRap may have a higher search volume but 'Rap' defines a genre of the culture, and the culture is Hip-Hop.
Major media including: BET, MTV, Rolling Stones, TMZ refer to the title as King Of Hip-Hop
http://www.rollingstone.com/music/news/introducing-the-king-of-hip-hop-20110815
When you type in "King of Rap" on Google, the first thing you read is:
OKP News: Rolling Stone Crowns
Eminem “King of Hip-Hop” OK, where to start with this one. As you may have read if you are a) a rap fan and b) get the internet, Rolling Stone Mag this week declared Marshall “
Eminem” Mathers as the official (!) “King of Hip-Hop.”
If either of us get an inquiry, let's just join forces and counter @ double.