There isn't an upside. Theoretically it means that certain investment vehicles must buy shares of your company. But they do so from the open market; not from offerings.

Otherwise, it's just a matter of prestige: the S&P lists are just the X largest (by stock value) profitable publicly traded companies.

The difference between S&P X00 and Nasdaq: S&P is just a list of publicly traded companies; it is not a stock exchange. Nasdaq is an actual stock exchange, like the NYSE.

Getting investment money from anyone who invests in an S&P500 index. Which is a lot considering how often the S&P500 is considered the default index.