At Vanguard's brokerage, the expense difference in VTSMX may cost you all of two dollars (sub-10k only; free, fast conversion to VTSAX when you break 10k). In exchange you get an easier conversion (you can go MF -> ETF if you want later without the potential tax hit) and free automation without mutual fund fees. I have also heard you get very very slightly better tax treatment via tax-loss-harvesting methods in the mutual fund, if a taxable account.
Whether you can automate, though, is a function of your brokerage when it comes to ETFs. We need to know the brokerage to help with that. If they charge commission on ETFs, you're almost certainly better going with Vanguard's brokerage, especially investing small, regular amounts.
Invest $100 for July
Lose $6.95 brokerage fee
Immediate loss: 6.95%
(Sure, there isn't an ongoing charge on the same $100, but a 6.95% tax on incoming money makes VTSMX fees look like Christmas in July).