It isn't that simple. Going to a C-M, even if you are 100% certain of being totally innocent, is a very risky endeavor.
A C-M doesn't have the same standard of conduct that a civilian court does. In a C-M, charges can be added, changed, or modified at any time during the proceedings, so basically once you open the door up, anything and everything that is discovered during testimony is in play. You can enter a C-M charged with one thing, and exit convicted of something else entirely depending on what came up during evidence and testimony.
Remember that the UCMJ does not include a presumption of innocence, and depending on the charges, has different standards of evidence and conviction than what we're used to in the civilian world.
Plus, a conviction at a C-M is a federal conviction, while an Art 15 isn't anything at all in the outside world.
Add all that up with the witch-hunt environment which we know exists in the USAF with respect to some topics (like sexual assault, particularly), and that is the makings of a potentially very bad situation for someone accused and being offered an Art 15.
During my career, I had the "opportunity" to pay a large chunk of money to two different, very well known and talented former SJAs (and now high profile civilian attorneys) and they both heavily, heavily suggested taking the Art 15 rather than risking a Court-Martial.