20k for an 05 with 70,000 miles is not a bad price, and it has one of the best versions of the duramax for power in alot of peoples eyes. The 2005 and 2012 is completely different so its difficult to compare the 2 trucks considering body style, different version of the motor, etc... Buy what you like and what your comfortable with. I havent seen a new diesel pickup thats worth over 50k yet in my opinion. Also the 2005 has less emissions than the new 2012 which is cheaper to modify.