Impact....by that definition its Em and its not even close.
Before the fangs come out, im not a huge Em fan, and I think biggie is the illest. Biggie is by far the better of the two, talent and catalog wise...but impact?
Hip hop popularity exploded concurrently with Em's rise in popularity. No coincidence. White America ( the overwhelming majority), finally had a rap hero they could call their own. Em brought in the "rap is crap, it aint music" honk demo, and it opened the genre up to millions of fans that other artists capitalized on....fans that I dont think would have been there if not for Em.
And say what you want about the value of such, but white rappers were still very much taboo in the late 90s. After Em, labels couldnt sign them fast enough, and within a year or two of Em's ascension the white rapper stigma was all but gone. Thats impact.
Em helped pull Dre off the career scrap heap, which fueled the aftermath comeback, and his co-sign established 50's dominance.