I did not visit both Schwartz's and The Main on the same day or even the same visit so this will not be a comparison except at 2 PM there was a line at Schwartz's and no wait for a table across the street.
Ambience - meh. It's an old diner like place, booths jumbled into the space, some tables out back, paint and Formica. Service - efficient but you shouldn't come if you want the waitress to become a Faceboo