You should really talk to a lawyer to figure out what exactly you want to be doing and how to best go about it as there are big differences.

For example:

• Working as a consultant you don't have to establish and register a Gewerbe (i.e. a "business") as some activities are considered "free trades" (freie Berufe) instead of a business, consulting being one of them. This has some implications, namely that income derived from a freier Beruf is not subject to business tax (Gewerbesteuer) but is taxed only at your personal tax rate and has to be declared as "Einkommen aus selbständiger Arbeit" rather than "Einkommen aus Gewerbebetrieb."

• The earnings in a business year will decide whether you have to add VAT to your invoices or can choose to do so. In case you're under the threshold and can decide whether to participate in the VAT system, your decision will really depend on what you're doing with whom and how much bureaucracy you will want yourself to subject to.

• ...

The above are only some of the many questions you need to answer before you make a decision and nobody here will be able to give you a good answer without knowing in detail what you're planning to do.