As an author for FA14, I can say that the FA review process is a balance between trying to make the text as comprehensive as possible (i.e., even including what would be considered some LY details) yet keeping it HY enough so that it doesn't take on the superfluous characteristics that Kaplan notes are notorious for.
Basically for everything added, something is removed or consolidated. And the edits are generally reductive rather than additive. This explains why an entry like the lac operon didn't make it past FA12. It's not that this entry wouldn't have made FA13/14 more comprehensive; but other stuff considered higher-yield was added at its expense.
I've been a member of this forum for over two years, and I've seen / been PMed with pretty much every story possible about "absurd, wtf info" that has shown up on the real deal. In turn, apart from just using my own experience/judgement, I had done my best to "vouch" for certain content to be added to FA, based on people's feedback on this forum.
The net result is that FA14 now has quite a few new "factoids" thrown in there that are actually floating around on the real deal.
Examples are SLE is associated with lymphoma (end of path chapter), and the definition of a chaperone protein (biochem chapter).
People will frequently come out of the Step1 saying that FA was mostly comprehensive. The reason it's not fully comprehensive is because students actually screw other students over.
FA wants to stay HY and be affable to as many people as possible, so we recruit large numbers of recent test-takers to vote on the new material going in (democracy in a sense); lots of really really good stuff that some of the authors "know" is on the real deal gets voted down if students don't like it, and it's annoying because it won't get added even though it really would increase people's scores. Examples would be methimazole causing aplasia cutis congenita, exogenous PTH in pseudohypoparathyroidism causing normal effects at bone but expected non-response at the kidney, the function of importin-alpha as a nuclear shuttling protein. This stuff is actually on the real deal, but was voted down and not included. You can thank your upperclassmen for that.
Regardless, I'd recommend FA14 over FA13. Even if you're content with your currently annotated copy of FA13, your friends who read the newer edition will have an advantage based on the new content.