how EXACTLY does a Solo 401k differ from a self-directed IRA?

[Rick D.]

Does anyone know where I can find a definitive guide on self-directed IRAs vs Solo 401ks. I mean something that goes beyond the superficial and tells me step-by-step EXACTLY what they are and EXACTLY how they differ and EXACTLY what they typically cost to setup and administer? I've listened to a dozen podcasts on this topic and no one ever goes into a satisfactory level of detail.