With both accounts, you will not be taxed on your investment returns while the funds are in the account. But once you retire, you will be taxed on your mandatory withdraw.
A Solo 401k is also known as a self-employed 401k. Full-time freelancers and self-employed individuals are not able to take advantage of a permanent employer retirement plan, so instead that can opt for a solo 401k.
Do you have to be completely self-employed to have a self-directed 401k? No. To open a self-directed 401k, you must have earned self-employment income.
Most employer-enabled 401k plans only have a small selection of investment options for workers to choose from. This means that an employer has a large say in your investment portfolio.
Individual stocks may not be offered in an employer plan for example. However, with a self-directed 401k plan, your investment options include anything a brokerage house would offer.