I ran some calculations on a self-employed retirement calculator, and as the chart below clearly shows, the Solo 401(k) is a superior retirement plan for the self-employed regardless of income level. The Solo 401(k) allows self-employed individuals to put more money into a tax-advantaged account while reducing taxable income better than any other retirement vehicle.
The one advantage the SEP IRA has, is that it can be opened AFTER the tax year has ended. A SEP IRA can be created as late as April 15th (or as late as October 15th if you extend your tax return) for the previous tax year. Thus, while the Solo 401(k) is the best way to go, you must open the 401(k) account before the end of your tax year (December 31st).
Being self-employed means you are both the employee and employer when it comes to retirement plans. You must create the solo 401(k) account before December 31st and make the employee contribution also before December 31st. However, the employer contribution can be made as late as April 15th the following year (or October 15th if you extend your tax return).
Self-employed individuals can contribute up to 100% of their compensation to a maximum of $17,500 (plus an additional $5,500 if aged 50+) for their employee portion and up to 25% of net income for the employer contribution up to $34,500 for a grand total combined maximum contribution of $52,000 for 2014 ($57,500 if aged 50+).