A Savings Incentive Match Plan for Employees (SIMPLE) IRA will not allow employees to contribute as much as they could with a traditional 401(k), but it does require an employer match.
Traditional or Solo 401(k)
Both of these 401(k) options provide a lot of versatility in terms of employees being able to access them for loans and employers being able to choose whether or not to match contributions. For employees over 50, they offer catch-up and Roth options that tax contributions upfront instead of at withdrawal. A Solo 401(k) is good for sole proprietorships or partnerships; if you have additional full-time employees, you won't qualify.
This is a totally employer-funded plan with no employee contributions; a SEP IRA does not allow for loans, catch-up contributions, Roth or profit-sharing options. The upside is that a SEP IRA has more relaxed tax reporting requirements and earnings grow tax deferred. Sole proprietorships, S and C corporations, partnerships and LLCs qualify.