I'd do the 2012. Sooner or later those DBs with get upgraded and new servers will come with SQL2012 installed on them. Other than the new features everything i the 2012 exam is still applicable to 2008R2, so why not get the newest? This saves you the bother off upgrading later. I'm thinking of skipping 2012 and waiting for 2014, but I already have the materials(had them for a year now, still haven't written), so I'll probably still do the 2012 exams.