While Vanguard was the first-mover in the low cost space and were the leader in low-expense mutual funds, they have since seen very strong challengers, primarily Schwab and Fidelity. Vanguard was not the biggest, but they were leader in the passive investing index mutual fund space. However, when the movement they spawned grew substantially, all the big players with broader operational capabilities got in on the action.Therefore, from both a plan design perspective and investment perspective, I would recommend Schwab or Fidelity.If you have opportunities to invest outside the stock market, you should consider self- directed Solo 401k Checkbook Control 401K Plans - 401K Checkbook or a self-directed SEP-IRA SEP IRAs - 401K Checkbook. Using a self- directed plan will ensure you get the best plan features and access to all investment types. With a self-directed plan you could get access to Fidelity and Schwab and invest in real- estate or private lending.Incidentally, it would seem more likely that you're considering a Solo 401k and a SEP-IRA (as opposed to a SIMPLE-IRA SIMPLE-IRAs - 401K Checkbook). A Solo 401k offers many benefits that are unavailable in a SEP-IRA, such as the ability for the business owner to make salary contributions, the ability to borrow from the plan, Roth contributions, and favorable taxation of real-estate investments.