I think I can shed some light on this subject. I will use an industry I am familiar with to illustrate. Many law firms these days are considered "small businesses." I live in a county with about 1.3 million residents, and some of the attorneys I am friends with refer to themselves as one of the following:
"Owner" (less common)
As far as the most popular designation, it's tough to say. Partners tend to use "Partner" or "Founding Partner," while attorneys who are founders of a non-partnership structured firm, such as a professional corporations or professional limited liability company, tend to use, "Principal" or, "Principal Attorney." A good number of solo practitioners also simply use (Esq.), but some attorneys do not think this is in good taste (I have no idea why not).
I myself am a founder/owner of a Virginia criminal defense firm registered as a Professional Limited Liability Company (PLLC). I don't go around telling people I'm "Principal so and so," with the exception being my email signature. It is: FIRST NAME, MIDDLE INITIAL, LAST NAME, Principal Attorney. Quite frequently, people who call my firm do ask if they are talking to a firm leader (or something similar). This is the only reason why I see any benefit using "Principal." It makes people more comfortable and likely to trust they are contacting a decision maker.