As you already know, there will be a new MCAT exam in 2015 that is more comprehensive and is a reflection of the changes in medicine and medical education. In addition to the natural sciences, it will also test social behavior sciences (psychology and sociology) and critical reasoning (with passages ranging from philosophy to population health).
So which MCAT exam will you have to take? To decide, take a look at this flow chart courtesy of AAMC:
If you are taking the new 2015 MCAT, read the preview guide for the new exam HERE.