If you get the A, take it and run, period. There are more applicants who are "good enough" for a top 20 school than there are seats in the classes.
Additionally, I do not believe that it would really be "gambling" to try and get a top 20 admission. An interview is not a slot machine where you pull the lever and then see what happens. For whatever reason after interviewing, 6 schools all decided to put you on the waitlist, and while it doesn't feel good I think one needs to conclude that while on paper you are a competitive applicant something went wrong at the interview stage. The problem may be something you don't recognize or that you can't change. So rather than throwing away a year's of attending salary hoping you can do better next year, just go to the school that takes you, kick butt, get into a "better" residency and build the career your want.
As an aside, you may well get into one of these top 20 institutions anyways.