Reliability of new 2015 6.7 powerstroke or Cummins Diesel

Your best bet for acquiring that information would be to visit the Dodge/Ram forums.
There you'll find the real problems.
As for the Ford, first year (2011) saw some problems which were largely corrected by the end of 2012.
Both trucks are outstanding. If buying used, I'd start with 2015 or later.