<prompt> Human: Im specifically wondering about the difference between Vanguards VTI ETF and VTSMXVTSAX mutual fund. My understanding is that these are the same index, but there are differences in how they are traded. I have read that ETFs are traded like individual stocks. Does this mean you can only buy whole numbers of shares? The ETF also has a lower expense ratio but the same as VTSAX admiral shares. What is the purpose of having this distinction between the ETF version and the mutual fund version of the same index? What accounts for the difference in expense ratio?If I am planning to invest 10,000, does it make the most sense to go with the mutual fund admiral shares for the simplicity and lower expense ratio?  <response>: Get the mutual fund. There is very very little difference between these for most investors, and doubly so if you are going with Vanguard and will hold Admiral fund shares:1. ETFs have theoretical advantages in avoiding some capital gains distributions, but the larger Vanguard funds are pretty good about avoiding cap gains distributions. It would require a massive number of people to sell lots of shares in these funds to trigger meaningful capital gains distributions.2. Mutual Funds allow you to avoid bid-ask spreads and make your purchase at the stated NAV. You can also avoid brokerage fees although they may be waived on Vanguard ETFs held on vanguard.com. If you arent planning to actively trade the ETF then I would avoid these costs and get the Mutual Fund.3. Mutual funds allow fractional shares and easy reinvestment of dividends... however when vanguard ETFs are held at vanguard.com, I believe those advantages to the fund disappear.4. Finally you can convert an Vanguard Mutual Fund holding into an ETF share. BUT NOT THE OTHER WAY AROUND.So for me I say: Get the fund, but the only real reason is 4. The others dont actually matter, and there is no meaningful difference. But 4 gives you the flexibility to change your mind in the future if conditions warrant. 