I recently had to select a 401(k) plan for our small startup. For a startup, the _employee_ fees was significantly better on Guideline (0.15 - 0.3%) than Fidelity (0.5% + $100 bookkeeping fee). The _employer_ fees were slightly more expensive with Guideline ($1,778 on Enterprise plan for Guideline vs $1,200 for Fidelity) but offered more features.

Important for founders in the US to know: you can put up to $70k annually into your 401k using profit sharing, which only some 401k plans offer. Your startup does not need to be making a profit to do 401k profit sharing. Employees may also be able to negotiate this!