I run a company with about 40 employees, and I am the 401k administrator. I'll share my experience.
When I joined the company, in 2008, I didn't like our 401k plan, so I shopped around. The plan was small at that time (maybe $1.3m). None of the big name firms were interested. I couldn't get Fidelity, Vanguard, etc. to even return my calls.
So, I went with ADP (our payroll provider). The selling points were: (1) they specialized in smaller company plans like ours, so they were willing to do it; (2) they offered low-cost index funds as well as higher-fee actively managed funds, in most of the core asset classes; (3) they integrated with our payroll pretty seamlessly (this helps from a compliance and record-keeping standpoint); and (4) the administration costs to us as a company were not much, and to the employees, nil.
Fast forward 4 years, and our plan had grown (about $2m last year, now probably $2.5). I take a closer look at the fees, decide to shop around, and it's a different marketplace. Fidelity is now interested in plans our size (both because they need to grow by adding more dollars, and so they're looking at smaller plans, and because our plan assets have grown). And, bonus, they offer a payroll service that, by my lights, is surprisingly equal to (if not better than) ADP, and the employer costs are no more. Given some nagging problems I'd had with ADP, and the fact that the overall fees and expenses to plan participants were notably lower, it wasn't a hard call to switch, and I've been happy in the year we've been with them. I think it's a better overall solution and employees seem happy with it.
Bottom line lessons: (1) lots of small employers want to offer good plans, but not all the brokerages want to go through the record-keeping hassles for a low fee -- but, at least in my experience, the marketplace has evolved over the last half decade. (2) payroll integration is a big bonus for employer hassles (this is why Fidelity is doing payroll now).