1. 2015 has coils in the rear.
2. 2015 has two fuel filters.
3. 2015 has radius arms instead of control arms.
4. 2015 has an updated interior.
5. 2015 has updated the front suspension.
6. 2015 has DEF.
7. 2015 has active air intake.

Price/miles aside, I'd 100% buy a 2015 over a 2012, assuming both were mechanically sound.