If you put in $5500 this year, you can buy VTSMX at a .15% expense ratio. You can do so using automatic investments and purchases. You can do the same thing next year. Once the balance exceeds $10,000, Vanguard will convert it to VTSAX at .04%. Over two years, your fee is between $16 and $20.
You can make auto contributions the same but cannot make auto investments in an ETF at .04% expense ratio. You will have to log in and make purchases and will have a cash balance after each purchase. The cost of the ETF will be between $4.50 and $7.
In other words, you saved a whopping $11 to $13 over two years by going with ETFs.