'15 and later got a redesigned interior, if it matters.
Read recently that the new '17's have the Apply Car Play and similar Android app available. I don't really care myself, but just something to add to the mix.
Personally (if it were for me), bag for the buck, I would look for a '09 or '10 SRT8 with a stick. Supposedly, the 6.1 isn't significantly faster than the 5.7, but no way you could replicate the SRT suspension and brakes on an R/T for the difference in cost, and I think the 6.1 still has more potential than the 5.7.
If cost were less of a concern, but still a limit, I would be looking for either a 6.4 SRT or Scat Pack with a 6M. I would think the SP would be similar in value to an '11 to '14 SRT, but the SP would have the new interior design.
For me, the 6.4 is the one to have, regardless of what it is in. Next step down would be a 6.1. I'd take a 5.7, but I would always feels like I settled. Just to be clear, though, this is from the theoretical "what-if" arena, not from someone that has either spent the money, nor driven either on more than a test drive. I have test driven both a '10 R/T 6M and a '16 R/T 6M and it felt like to me that the '10 was faster, and that neither would satisfy me for very long. Didn't drive the '16 SP as I knew I would get out of it and sign papers and wasn't ready to take that step.
I think about this often, unfortunately. I also avoid the dealership and Autotrader online when I can.