The one advantage I can think of is that traditional solo 401(k) contributions can reduce the QBI deduction while Roth contributions don't, so you effectively only get 80% of the deduction from traditional contributions unless you have some other factor reducing your QBI. This doesn't mean you shouldn't contribute to traditional solo 401(k), just that you should compare your actual marginal tax savings to your expected marginal tax rate in retirement.
You can make Roth employee salary deferral contributions at some solo 401(k) providers, but while the the same law that allows Roth SEP contributions allows Roth 401(k) employee profit sharing contributions I'm not aware of any free/mainstream providers that have amended their plans to allow it at this point.
As far as I know you can't "maintain" both a solo 401(k) and a SEP IRA in the same year. For SEP "maintain" means contribute to. I'm not sure if a solo 401(k) is considered maintained whenever it's open rather than only when contributed to like the SEP, but it wouldn't surprise me. I would tend to prefer the higher limits of the solo 401(k) unless you have high enough self employment income to make the max contribution to either on profit sharing contributions alone.
