Calling them "rival genres" implies they're vying for the same audience, and they're clearly not. If anything, hip-hop and pop would be rivals as they're both more mainstream and most rock, especially anything metal. As much as we love metal here, I think it's safe to say it doesn't get as much of the public reach as hip-hop or pop music does world wide.
I think metal and rock fans mostly don't gravitate towards hip-hop because rock fans are usually instrumentalists who enjoy rock for the musical and instrumental parts of the genre, where as hip-hop is more focused on image, feel and flow, and less on instrumental composition, more in lyrical and thematic content.
But there have been plenty of crossovers here and there, much to the dismay of fans on both sides, but to call them rivals seems very misleading. Personally, I don't dislike hip-hop and in fact it has its merits, I just prefer rock and metal. However, I'd take both over the increasingly uncreative country music ANY day...
-Marc.