It looks like contribution limits for employers are better with solo 401k, and 'employee' contributions (wife?) can be higher and done as a Roth.
The killer for many plans are the employees, who legally require contributions according to pretty generous vesting schedules, and this also increases costs of plan administration. It also places a 'contribution' tax on the payroll that can negate the benefits of the plan to the employer. That, and employees seldom recognize or appreciate the contribution, but become a leg of harassment over loans etc. from the plans. again, administration costs and hassle. I have known small business professionals who have foregone the more complex plans for their own retirement for these reasons, and just have personal IRAs. If you have very high turnover of employees, it might still be OK according to cost benefit analysis, since they may not qualify or vest, and a portion of their contribution would revert to the general plan fund i.e. owner and long term employees. Still, plans have administrative costs that may not be practical for small businesses.
For solo business with wife, there is no such disadvantage if you want to make large contributions.
I have a SEP. Worthwhile for me because I'm Incorporated as a C Corp and have been heavily front loading my retirement as the contribution limit is quite high.
SEP - contribute up to 25% of the salary. (2018 max $55,000)
solo or i401k - has two parts to contribute: (2018 max $55,000 + 6k if you are 50+) 1. Employee Contributions: As an employee, you can defer up to 100% of your compensation, or “earned income” up to $18,000 (plus $6k if you're 50+ = $24k max) 2. Employer Contributions: This is a bit complicated to calculate, its anywhere from 20% to 25% depends on your situation (irs.gov site has a form to do math) btw, if the account total value goes more than $250k, irs expects to fill certain form(s) periodically and submit to clients I guess.
(Also, long time- no see!) :)