Ah, courses and not exams. I would take the 2008 course if you have no choice to take the 2012 course. There are hardly any features available in 2008 that do not translate perfectly to 2012 that I can think of save for maybe Database Mirroring and those changes are minor. I would take the 2008 course and then supplement the 2008 course knowledge with the new features added in 2012 using Books Online, online tutorials and maybe a purchased text or two and go for the 2012 exams.
There are no special teachers of virtue, because virtue is taught by the whole community. --Plato