A Solo 401k offers a business owner the ability to use his retirement funds to make virtually any type of investment tax-free, including real estate, on his own without requiring the consent of a custodian. The IRS only describes the disqualified investments, and very few at that! All the income and gains goes back to your Solo 401k tax free!
An IRA allows you to contribute $5,500 a year, or $6,500 if you’re over age 50, to “catch up.” The problem with this is that you can only get $100,000 into the plan over 20 years. That’s just not enough to get close to what you’ll need for retirement.
With a Solo 401k you can contribute $53,000 per person, up to $106,000 with a spouse and even more if you’re over 50. ($59,000 each in 2016, if over 50 years old)