All provide similar benefits and at similar rates - see rates below. The choice of which one to take really depends on factors such as administrative effficiency and available extras.
Public Insurance is generally only available for employed persons earning a salary within certain fixed limits. Public Insurance is sometimes available to Self-employed (Freelance) on a voluntary (Freiwillige) basis BUT only to persons who are moving directly from an employed position to self-employed (freelance) status, or immediately and directly from another EU Public Insurance - such as the UK NHS.
Spectrum can provide advice and information and arrange public insurance where appropriate. The premiums are calculated purely on salary and are therefore the same wherever the insurance is arranged. Spectrum do not levy any additional fees or charges.