i dont think you will find much of a difference between 2012 or 2013. i do have a 2012 and my parents a 2010 which is a very big difference though. 2013 will probably be best as they would have sorted out any "issues" the 2010-12 would have had. My 2012 has been nothing short of amazing after 5 years of owning it and 65k miles. I do maintain it up to Mercedes's manual and the engine drives as good as it did in 2013 when i got the car. Also to note i dont have a 4matic it is RWD.
pros:
Everything about the car just feels right. The power, handling, ride quality, transmission, and braking are all smooth and all feel paired together well. Truly a car you wont regret buying.
cons:
I do kind of wish sometimes I had a bigger engine (E550) or a smaller car to park easier. both not really bad though. this car was #1 in its class for many years for a reason