As of right now, the AOA IM Exam is cheaper than the ABIM IM ($800 vs $1385 respectively)
As of right now, I don't see the benefit of being AOA certified over being ABIM certified ... unless you want to be a dean of a DO school (requires being AOA board certified)
ABIM is the exam that all MD internists take (from FMG heavy community hospital program to Hopkins Osler's program, MGH, etc). Your certification status with ABIM is easily confirmed on their website, all the test prep books and courses are geared towards ABIM, you don't have to study OMM for your board certification, better written questions (so I've heard, etc)
Your ABIM board certification status can be easily looked up on their website, by your hospital credentialers, insurance panels, patients, etc.
Now go look up someone who is AOA certified.
As of right now, in order to maintain board-certified status - you MUST be an AOA member in good standing (there is talk about de-coupling AOA membership and board certification, plus there is an ongoing lawsuit in regards to this requirement). If you let your AOA membership lapse (ie not enough AOA 1A CME credits, not paying dues, etc), you lose your board certification status. Depending on where you work, that may halt your clinical privileges (depends on the bylaws)
*disclaimer -potential conflict of interest. I hold multiple ABIM certifications.