@Dave Versch
Unless you are over age 59 1/2, you cannot draw any capital from a self directed IRA or 401(k), and then only by taking taxable distributions. These plans are for retirement investing only and there can be no direct or indirect benefit to you personally.
The decision between IRA and 401k hinges on a lot of factors specific to your situation, employment status, funding type, investment goals, etc. Speaking with a qualified advisor is the best means to make this determination.
You have to be self employed personally in order to establish a Solo 401k. The investment activities you plan to engage in with the Solo 401k do not classify as that self employment. For many people, a 401k is not an option, or if it is thanks to secondary self employment that is limited in nature - does allow for them to take advantage of the higher contribution limits - which must come from self employment income.
The key down-side of a Solo 401k is the need to maintain the qualifying status of being self employed with no full time employees. If this will not be a long term situation, then you might need to terminate the plan when you grow your business and add employees or shut down your business. Otherwise, yes, there are some advantages as compared to an IRA based program. What we find, however, is that a lot of folks do not really qualify, but have been swayed by their reading on the internet that the Solo 401k is the best things since sliced bread and really want to go that way. The best plan in the world is no good if it does not fit your situation.
If you are legitimately self employed and have a full time job with a 401k, your combined employee contributions will be capped at $18,000 if you are under age 50 or $24,000 if you are 50 or older. Employer profit sharing contributions and the plan maximum are not impacted by participation in two plans.
Both an IRA and 401k that are self directed are a fantastic way to diversify your investments into real estate and related non-traditional assets.