Complex sentence: A Solo 401k plan can be made in pre-tax or Roth (post-tax) formats, whereas, in the case of a SEP IRA, contributions can only be made in pre-tax format.
Split into simpler sentences: A Solo 401k plan can be made in pre-tax or Roth (after-tax) format. Whereas, in the case of a SEP IRA, contributions can only be made in pre-tax format.