I don't like reinvesting dividends - I want to use the cash to buy whatever lags behind in my portfolio. If you like reinvesting dividends, mutual funds allow that but ETFs don't.
Mutual funds tend to be expensive if you buy outside your current brokerage. If you're at Vanguard, you might have to pay a fee to buy Fidelity mutual funds. With ETFs, they trade on the open market like stocks, so they tend to be cheaper to buy than other brokeage's mutual funds. Also, most brokerage have a family of ETFs they let you buy/sell for $0/trade.
If you want to move from one asset to another, ETFs are faster: you sell one ETF, and gain a credit. You immediately use that credit to buy another ETF. (With mutual funds you wait for the cash to settle, then buy another mutual fund). Your cash spends seconds in cash rather than a day (longer if you forget to buy a fund the next day).
But the key thing is to invest, not delay while deciding between ETFs and mutual funds. If you mostly want to buy mutual funds, ignore my biased view (I like ETFs) and go with mutual funds. Starting is more important than a perfect decision.
Even a bad mutual fund beats cash.