they affect the IRA Roth conversion pro rata rule
Can you explain that more? I'm new to that rule.
I’m sure there are other disadvantages.
I believe with the SEP, you can only contribute 20 or 25% of compensation to the IRA. With the Solo 401k, you can contribute 18.5k + 20% of compensation. (Then both capped at like $54k). For someone making, say, $30k/yr in self-employment income, SEP allows you to put in 6k to 7.5k, and solo 401k allows you to put in 18.5 + 6k = 24k (though slightly less for math rules reasons I don't yet understand). I was going to go with solo 401k just because I'd rather tax-defer $24k than 6k, but I barely know what I'm doing.