When prices changing rapidly during the day, buying an ETF locks in the price right now, while buying a mutual fund waits for the close of the market, and gets whatever price is current at that time. If you're worried about the market's change in value from any given time until the next close of the market, ETFs are a better choice. (But note I'm biased in favor of ETFs, so try and find a contrary view on why mutual funds are best).
With both mutual funds and ETFs, the cost to buy can vary. When you try to buy Vanguard mutual funds from an account with Fidelity, it will cost a fee. Buying Fidelity mutual funds at Fidelity doesn't incur a trade commission. The major brokerages all have a family of ETFs which cost $0/trade, so it's worth knowing those.
Because of a change this year, at Vanguard about 1800 ETFs cost $0/trade. You can buy Vanguard, iShares, S&P SPDR, Schwab ETFs - all $0/trade. I'm not sure how Fidelity and Schwab will respond to that challenge, but hopefully by lowering the price they charge per ETF trade. But until they do, Vanguard is the lowest price place to trade ETFs.