They're both brokerage accounts. For the "Cash Management" account, you would earn interest (currently 2.21%) on cash that is "swept" to various FDIC-insured banks, whereas for the other type of brokerage account you would earn dividends on cash in a Fidelity money market fund (probably SPAXX, currently yielding 3.95%).

You can buy T-Bills in the same manner with either type of brokerage account, so choose whichever one fits any other needs you might have.