It's been asked quite a few times. A few points:
Reasons for Mutual funds: Most importantly, mutual funds can be automated to keep you, the weakest link, out of the system. Also, they don't have spreads and can be bought down to the last cent. For illiquid things like certain bonds there are arguments that the market price of an ETF can vary widely from the underlying assets, especially in a crisis, which makes the mutual fund better. Mutual funds transact at the end of the day when the price is on average a little higher, which might add a small preference to people who need to sell regularly.
Reasons for ETFs: They are more flexible for tax purposes and just generally more maneuverable. You can easily transfer them between brokers in pursuit of broker sign-up bonuses. Vanguard has a 60-day no-buy-back rule for mutual funds which is super irritating for capital gain/loss harvesting, but ETF's trade freely. Non-Vanguard mutual funds have unfavorable tax treatment, which would make you tend to prefer ETF's at Schwab or Fidelity for taxable accounts in any case. You are buying earlier in the day when, on average, prices should be lower and the additional ups and downs should also benefit regular purchasers. There are more stupid things you can do with them (note: stupid things are stupid).