IVV also tracks the S&P 500 directly, with essentially the same holdings and weightings as VOO and SPY. Its expense ratio matches VOO's at 0.03%, so long-term performance differences between the two are negligible.

In practice, the choice often comes down to which ticker has better trading volume or currency-conversion convenience on your particular brokerage platform. If you already hold VOO, there's little practical benefit to adding IVV as well.