The ETF expense ratios are lower, but you'll pay a couple basis points of spread to buy and another couple basis points to sell[1]. If you have enough money (t

The ETF expense ratios are lower, but you'll pay a couple basis points of spread to buy and another couple basis points to sell[1]. If you have enough money (typically $10k) to invest, then you can buy the Admiral Shares of the mutual fund, which has the same expense ratio as the ETF, but won't require you to pay a bid-ask spread to buy or sell. If you start with the Investor Shares and end up with more than $10k in the fund eventually, you can always convert them up to Admiral Shares without tax implications. You can't convert ETF to Admiral Shares without selling though. If you don't think you'll have $10k in the fund anytime soon, then the ETF is likely the cheaper option. However, the difference is practically really small - for a $9000 investment, you'll pay $4.50 per year in expenses for the VTI ETF vs. $14.40 for the VTSMX fund. [1] https://advisors.vanguard.com/VGApp/iip/site/advisor/investments/bidaskspread https://advisors.vanguard.com/VGApp/iip/site/advisor/investm...