For most investors, it probably doesn’t make sense to own both. VTI and VOO both provide great diversification at a low cost. If you hold both in your portfolio, you’ll have a lot of overlap between the two. However, you may find that your retirement plan at work doesn’t offer a total stock market index fund like VTI. In this scenario, if you open an IRA on your own, you might decide to invest in VTI on your own and an S&P 500 fund through your employer.