Unless you're pushing $300k of business income or have a regular job with a 401(k)/403(b) you contribute to a solo 401(k) will offer a higher limit than a SEP IRA. You might not want to open one if you plan to hire employees, but as long as your business is just you (or you and your wife) then a solo 401(k) is the way to go.
You might want to do some research on the new section 199a Qualified Business Income (QBI) deduction that was introduced with the 2018 tax bill. It gives you a tax deduction equal to 20% of your business income (QBI) if you qualify, but tax deferred contributions to a self employment plan aren't counted towards QBI, so you get a deduction equal to 100% of the contribution, but also loose 20% of the contribution in the form of lost QBI deduction. This essentially means your marginal tax rate savings on these contributions are 80% of what your normal marginal tax rate is. 12% becomes 9.6%, 22% becomes 17.6%, etc. For this reason if can be beneficial to contribute to Roth solo 401(k).
If you currently, or might someday, make too much to contribute directly to a Roth IRA and want to be able to make backdoor Roth IRA contributions (which previous traditional IRA balances complicate) another advantage of some solo 401(k)s is that they accept income rollovers from traditional IRAs which then lets you make backdoor Roth contributions without existing traditional IRA balances.
For these two reason not all solo 401(k) providers are created equal. Some allow Roth contributions (Vanguard, Etrade, TD Ameritrade) and some allow rollovers from IRAs (Fidelity, Etrade, TD Ameritrade). If you want both features, that points to Etrade, TD Ameritrade. I recently moved my solo 401(k) from Fidelity to Etrade so I'd have access to Roth given the QBI deduction considerations I posted above.