Hi Tivity606:
while both plans can be used for the self employed there are advantages and disadvantages for both. see the attached link article regarding self-employed retirement plans.
https://www.irs.gov/retirement-plans/retirement-plans-for-self-employed-people
i find the simple ira is very easy to set up. if you are a single member, than the 401(k) plan would be a solo 401(k) plan.
you might also want to consider a sep plan which allows up to 25% of net earnings. the article link attached addresses both plans. Its all a matter of personal preference and how much you can afford to put aside.
hope this helps.