I guess what year to get is a personal preference depending on what your future plans for the car are.

Are you going to leave it stock or modify it?

If the future plans include a supercharger a 6.1 is a better choice, otherwise go with a 6.4.
A little research will show you that.

My car is a 6.1 with a Comp Cam, kooks headers with high flow cats and an Arrington 85mm throttle body.
Obviously it's been tuned (HP Tune) and it did 417 rwhp so at that least puts it on par with a stock 6.4 (from their math it's supposed to be 490 flywheel roughly).
Someday when the budget allows I want to put a supercharger on it.

Personally I don't care for all the newer dash electronics on the 2015 and newer, I do love the newer interiors more.

I newer would have thought I'd see the day where the size of the touch screen in the dash is a selling point.