With the new SCR system and DEF they are supposed to be a little more friendly on the short DD's, that said even the older ones liked to be worked every now and then. The new ones even more so.
You have to drive them and make up your mind. The 6.4 is like any other large Gas engine, you hit the throttle you go, you hit a hill and you downshift. The Diesel you hit the throttle and you have your customary turbo and fuel lag coupled with some torque management then you go, you hit a hill it stays in gear and powers through it. Its a very different drive. I drove the 6.4 and had a big smile on my face, I drove the 13/14 CTD and while I had a smile I had some issues with the throttle response and go off the line. I drove a 12 and its in my driveway and I basically got the Diesel Upgrade for free by going used.
Sorry for the ramble but they are very different trucks (if your OCD like me), go test drive them for a while. See what YOU like, and remember do it right the first time or you will be shopping again.
I don't think you can go wrong with either, I'm a fan of the 6.4. I just can't not have a CTD after 11+ years of driving one.
Good luck, Rye.