I have been purchasing T-bills from TreasuryDirect, but I recently realized that I can also purchase new issue T-bills from Fidelity. The UX is better and when the money is between bond auctions, it can sit in the Fidelity money market rather than my low-interest savings account.
Also, TreasuryDirect has 4 week, 8 week, 13 week,... maturity but Fidelity has only 4 week. I suppose this could be the one benefit of using TreasuryDirect, but it doesn't make sense why Fidelity would not include these options.