VOO and VTI are  two of the most popular U.S. stock market ETFs out there. Both are from Vanguard.

VOO tracks the S&P 500 Index. VTI tracks the CRSP US Total Market Index.

As such, VOO is entirely large-cap stocks, while VTI includes small- and mid-cap stocks.

Specifically, VOO comprises roughly 82% of VTI by weight.

Consequently, VTI has been – and should be expected to be – slightly more volatile than VOO.

Since it contains small- and mid-caps, which have outperformed large caps historically due to the Size factor premium, we would expect VTI to outperform VOO over the long term, and indeed it has historically.

VOO has roughly 500 holdings and VTI has roughly 3,500 holdings, so VTI can be considered more diversified.

Both VOO and VTI have the same expense ratio of 0.03%. VTI is much more popular than VOO. I'd go with VTI.