With the ETF (VTI), there is a bid / ask spread. That is, you pay a little bit more than it is worth when you buy.
With the mutual fund (VTSAX), you pay the asking price at the end of the trading day. No bid / ask spread.
That is my understanding, but I've never actually bought an ETF (for reasons mentioned above).
It remains true that Vanguard tends to charge the same expense ratio for the ETF as the equivalent admiral fund. Investor shares of the mutual fund are more expensive, so that is a strike against the mutual fund if you have limited funds to invest.
Best,
PoF