ETF vs Mutual Fund DCA True Costs

Now that fractional shares of etfs like VTI are readily available, does anyone have any data on what is more cost effective long term for periodic dca investing? An index etf vs index mutual fund, assuming same expense ratio. Mutual funds are a known thing that settle at the end of day price. But for buying and automating VTI things are routed as market orders, so you are constantly paying the bid ask spread. And then it also depends on the order fill quality of your broker. Some are better than