I was with Vanguard for many years. Fidelity has too many things I like to split between two brokerages. Using 4-week treasuries on auto-roll is close enough to VUSXX for me. You really can't go wrong with Fidelity, Vanguard, or Schwab if all you need is a brokerage. But Fidelity has a much broader set of products that it offers compared to Vanguard.

https://www.bogleheads.org/wiki/Fidelity:_one_stop_shop