10K invested in VFIAX for the last 5 years is now $18,950.
10K invested in VOO for the last 5 years is now $18,950.
(VOO hasn't been around for 10 years, so I couldn't do that comparison. I used schwab.com as fidelity "didn't recognized" VFIAX.)
I didn't imagine that the returns would be within pennies. It's the return (and risk) that matters, not the management fee. Rarely does the cost to purchase matter--but in this case it does. There might also be a cost for redemption for VFIAX?
As for the differences between a low-cost mutual fund and ETF, the most compelling one I know of is that mutual funds trade at the end of the day so you can't take full advantages of flash crashes. It also means that if you want to transfer money from the mutual fund, there are potentially 2 or 3 days where you won't know how things wind up (sell, wait for processing at end of day, possibly one day idle, then buy at end of next day).
OTOH, mutual funds trade only at the end of the day, so you're protected somewhat against impulsive buying and selling.