My understanding of Vanguard "Admiral Funds" is that they charge lower management fees (have lower expense ratios) in exchange for there being a minimum investment amount.
However, it seems that currently in some cases the Admiral Funds version of a fund actually has a higher expense ratio than its investment fund counterpart. For example, Vanguard 500 Index Fund Admiral Shares (VFIAX) has an expense ratio of 0.04% whereas Vanguard S&P 500 ETF (VOO) has an expense ratio of 0.03%. The former is a mutual fund and the latter is an ETF. But, the only real other difference between the two as far as I can tell is the $3000 minimum investment needed for the former.
So, what am I missing? With the current expense ratios why would one ever want to buy VFIAX over VOO? (Note: from here it looks like they used to have the same expense ratio.)