First, let's get one thing out of the way: grime is NOT a sub-genre of hip-hop. While this should be obvious to Londoners who've seen the genre evolve from garage-rap to its current state, Azealia Banks is far from the only American who's gotten this key fact twisted. It doesn't help that even grime ambassadors like Dizzee Rascal have buckled under pressure when pressured to acknowledge that grime is basically "a sub-genre of hip hop."
I don't necessarily resent Dizzee for glossing over the details in this interview—for one thing, he still explains genre politics better than I can spit bars and he's got records to sell, so cozying up to backpack gatekeepers like Rosenberg isn't a bad strategy. Nevertheless, that doesn't change the fact that grime isn't hip-hop.
The story of London music is far longer than can be covered in one feature, but at its simplest, grime developed through the collision of a number of elements pulled from various genres and countries. Jungle's rhythmic energy and emceeing culture was itself a continuation of dancehall's live tradition and patois, and a younger generation then merged these concepts; garage's beats and tempo and hip-hop's emphasis on storytelling and street culture. Conversely, England has long produced its own hip-hop scenes, from the backpack-oriented UK hip-hop of the '90s to millennial road rap and more recently London drill by crews like 67. That's not even touching London's longstanding dancehall scene, which is another standalone entity.
To outsiders, it's easy to conflate anyone spitting with an English accent as practicing a variation on hip-hop, just as it isn't always obvious how jungle's manic dance music or dancehall's sensibility is directly linked to grime's gunman bars. If anything, these twisting links are what make grime culture such a wellspring of innovative music, and it's this synthetic approach that marks it as neither hip-hop nor dancehall, but its own art form entirely.
But let's put that conclusion aside for a second for a quick thought experiment. We chose six criteria on which to evaluate grime, contrasting dancehall's influence with hip-hop's to see which had more of an impact, historically and today: