ETF is good if you want do a one time big investment and leave it alone. MF
is good if you want to build up your position over time, because you don't
have to pay commission each time you add more money into it, which is
required by ETF.
Both index MF and ETF have the unique benefit of tax efficiency, that is
they distribute little capital gain along the way.
So it is recommended that you invest in index MF or ETF in a taxable account
. Roth IRA is tax free, so the tax efficiency advantage of in