We have a simple IRA and are just switching to a independent 401k because there are two of us so this may be irrelevant to you! But, here's some info anyway! Since we're co-owners with no employees, both allow us to invest for ourselves (or just one of us, based on who's ready) because we have no employees. If you have employees, you can't do these kind.
The 401k allows up to $18000 yearly "employee" (that's me!) input and up to 25% salary match from "employer" (me too!) up to a total per year of $52k approx. All pre tax!
The simple only allows up to $12500 per year - I did this last year. Pretty easy/straightforward except you cannot contribute to any other IRAs for the year you do the simple and I think even the next calendar year too (meaning if you do a back door roth - where you contribute the $5500 I think it is to the traditional IRA post-tax and then transfer it tax-free to a roth which you can use someday tax-free).
For us the SEP was not an option because we are not solo.
One cool note, the 401k allows "spouses who are employees" to also get the same retirement savings benefits, but the MINUTE you add an employee you lose the benefits otherwise - go micropractice!