One other advantage of ETFs is that they are slightly more tax efficient.
The main disadvantage is that, unlike a mutual fund, they can cost more or less than the net asset value of the funds in the index because they are actively traded through the day. For a buy and hold investor this is a non issue.
These days you can purchase most plain vanilla index etfs through your brokerage fee free.
So the final advice is, when choosing between an ETF or the mutual fund which it tracks, choose whichever is cheapest.