Both were lyrical geniuses, which is why their music has stood the test of time (keeping in mind that hip hop is a very young genre, especially compared to something like rock). Both were innovators, pushing the boundaries of their styles. Tupac was certainly a great storyteller. He was a legit thespian after all. His popularization of the narrative has spread throughout all of modern hip-hop. Much like Shakespeare was given credit for establishing the archetypes of modern literature, Tupac has established the archetypes of modern hip-hop. Unfortunately, a lot of the guys today are all theatre and no substance. Tupac was speaking from the heart and being creative, while others today are attempting to be cheap imitations that are looking to cash in for a quick buck.
Biggie, on the other hand, had a much rawer set of tools, which is why you hear him go to some very dark places in songs like, "Dead Wrong." The stuff that really pushed the boundaries for him was his less commercial work, and you can certainly hear his influence today when you listen to the hardcore scene. Guys like Vinnie Paz wouldn't have careers at all if it weren't for Biggie. His very "from the streets" approach has also influenced modern hip-hop, creating the pathway for many of the "rags to riches" ideas that come up today. Unfortunately, many of today's stars are just actors, their stories of the streets being extremely inauthentic. Few have something new to say, unlike Biggie.
In the end, modern hip-hop owes its entire being to these two. Because of his theatricality though, I have to give the nod to Tupac.