I’ve been self-employed for nearly 20 years and over those years have had time with both plan options.
First, I assume you have no employees, correct? If you do, they would almost always need to be included in the plan(s).
SEP’s are nice because they are easy to set up and cheap/easy to administrate. 401k’s do require a bit more work, but places like Vanguard can set up a solo-401k type plan for $0 (or at least they did when I set my plan up). You can pair the 401k with a profit sharing contribution, too.
Depending on your income, 401k’s often allow a bigger contribution. For example, if your company is an S-Corp and you pay yourself $50,000 on a W-2. A SEP is limited to 25% of the $50k, or $12,500. With a solo-401k, you could defer $18.5k or $24.5k depending on your age. Plus, the company could make a profit sharing contribution of 25% of the $50k, or $12,500. So the total contribution could be up to $31k (or $37k if 50 or over). The SEP and profit sharing contributions are discretionary, so they are not required each year (just in case the profitability is low one year).