There are many factors to consider.
Vanguard offers Admiral shares of their S&P 500 VFIAX with a minimum investment of $10000 which has a .05% expense ratio, or you can buy VOO, an ETF with a .05% expense ratio. VFINX can be bought with a minimum investment of $3000, with an expense ratio of .17%, but you can get that discounted if you use electronic statement delivery through Vanguard, at which point your expense ratio is pretty close to the admiral shares. If you are using some provider other than Vanguard, you may also pay trading fees to buy Vanguard funds. You also have to wait until the end of the day prices for trades, so you won't have the protection of stop-loss orders if you want to trade in a volatile market.
If you are buying from another broker, you will often pay fees to buy and sell ETFs, but if you have a Vanguard account you can buy Vanguard ETFs (like VOO - their S&P ETF with a .05% expense ratio) without paying a fee. Even with a Vanguard account, you can only buy whole shares of VOO, so you can only invest in increments of ~$186 (the current price at the time of writing).
So to summarize, you can invest better granular amounts with a mutual fund, but you have a large minimum investment if you want the lowest expense ratios, with only day ending prices to trade on. You can buy the ETFs directly, but you have to factor in exchange fees to your expense calculations if you are paying those fees. If you are paying those fees, then your incentive is to make larger and less frequent investments, and even if you aren't paying those fees, you can't necessarily trade even-dollar amounts.