I prefer ETFs, mainly because I like to sell/buy in the same day (yes, I know I'm not a strict buy/hold person. Mutual funds are settled at the end of the day so it is easier to sell/get money next day vs ETF where you have to wait about 3 days for it to settle before withdrawing the money.
As I get nearer RE, I'll most likely transition to mutual funds for that reason, but until then, I'll stick with ETFs.