Provided your broker offers both solutions for the asset class or index you are trying to cover it comes down to costs, when you decide which instrument to employ.
Mutual funds are usually cheaper to buy, carrying a load of 1 - 2 % of the value involved for the purchase of the fund. On the other hand they tend to be slightly more expensive regarding the management fees than ETF's. Mutual Funds are usually better if you are dealing in smaller sums, and/or adding on regular intervals. See also Cost averaging
ETF’s are bought and sold like common stocks, incurring brokerage fees (courtage) both for purchase and redemption. They are cheaper in management fees, mainly due to their different set up. ETF’s might be better if you put large blocks of money into the market at a time, and if you can withstand the temptation of frequently trading.