Mutual funds are simpler - your buy/sell order happens upon the close of the stock market, at the closing price. A disadvantage is that when you want to sell one fund and buy another, you need to wait 1-2 days between the transactions (close of market, funds settle, buy at close of another day).
I like ETF flexibility for rebalancing. For example, selling VTI then immediately buying VXUS. The "unsettled funds" from one sale can be used to buy ETFs in another sale immediately.
But the bigger problem is indecision. Most people in this thread are saying mutual funds, so I'd echo that opinion. Go with mutual funds just to get started. The difference between ETF and mutual funds matters far less than getting started.