Where should I park emergency fund cash (that isn't in T-bills)?

Looking for opinions: For the last several years I have kept +/- six months of living expenses in HYS and T-bills, HYS is for instant accessibility needs and the T-bills are for slightly longer or larger needs. It looks like Vanguard's VMFXX sweep account and Cash Plus account both pay a higher interest rate than most HYS so I'm thinking of moving my HYS to one of these. Any thoughts on this? Looks like VMFXX pays slightly more but the Cash Plus account is FDIC insured. Is there anything I am missing?