I think Snoop signing with Death Row was more significant because while Snoop was the biggest star on Death Row, Dre didn't need Snoop to validate him as a great producer. Dre was able to groom Tha Dogg Pound and Pac into being multi-platinum acts.
But Biggie is the biggest star that's ever been on Bad Boy and also validated Diddy's talent as a producer / star-maker. I know Craig Mack's album was the first album on Bad Boy to drop but without Biggie, Craig Mack, the LOX, Lil Kim, Black Rob and the rest of the don't get the notoriety they reached.