If you're a sole proprietor, it would be pretty silly to call yourself a "Sr Consultant" --- Sr compared to what?
Principal is in fact a title used for Consultant/Owners.
Principal Consultant, on the other hand, is usually the next step after "Sr Consultant". Unlike "Sr Consultant", you will not sound goofy calling yourself that. However, if your consultancy grows and you need to create a ladder of titles, you may want to reserve Principal Consultant for employees.
(I hate that Principal Consultant has that meaning; it makes no sense to me that the owners of the firm are "Principals" and the most senior delivering consultants "Principal Consultants" --- what are they, the Presidential Guard? But that is how it works.)