Yeah, very different product. Muni bonds are riskier than treasuries and much longer term than money market funds / treasury bills. E.g., VWSTX ("ultra short term muni") has an average maturity of over a year and 45% of portfolio is worse than AA. Vs VMFXX (money market) with an average maturity of 11 days and the entire portfolio is backed by the US government, which is essentially as good as it gets.
My impression is that the current yields of these funds are not high enough to justify for most taxpayers, compared to other funds. But since everyone is different, it can be helpful to sit down and do the math for your individual situation.
https://thefinancebuff.com/best-vanguard-money-market-fund-y...
* Schwab's roboadvisor product has decent rates for cash sweep, but their self-directed brokerage has the rather weak rate of 0.45%.
* The first $10k at IBKR earns 0%. Also, IBKR Lite also has a weaker rate (3.83%) compared to IBKR Pro (4.83%).
Of course, with a bit of extra effort, you can buy SGOV etc. in your brokerage for better rates.
[1] https://www.schwab.com/cash-investments
[2] https://www.interactivebrokers.com/en/accounts/fees/pricing-...
Marcus is FDIC insured savings though, which has slightly different implications. There's websites that compare current highest savings rates (nerdwallet, bankrate, whatever).
Going in the other direction, there are things like SGOV or Treasury-only money markets, if you live in a state that has higher income tax. Since the treasuries are exempt from state income tax.
Going even farther than that direction, there's muni bonds, funds, or money markets. At which point you're probably back to Vanguard for the best rates. (But again, not the same exact characteristics as a savings account)