As others have said the 6.7L has been a reliable engine with good driveability/performance.
I have owned Ford's with 6.0Ls, 6.4Ls, and 6.7Ls and the 6.7L drives circles around the two International diesels. Relative to a 2013 vs. a 2015, unless you are at altitude you probably will not notice a big difference in performance between a 2013 and 2015 for everyday driving. The 2015 does have more torque and a larger turbo for performance at altitude so if you load the truck to its maximum capability or frequently drive in Colorado you may notice a difference in performance, but in my opinion not enough to justify the additional cost of a new 2015 over a used 2013.