The difference is that the 2015 product has the 15.0 Fortran compiler and the 2016 product has the 16.0 compiler. Many new features, many optimization improvements and many fixed bugs. Your question "especially for CFD" is not answerable. I will comment that upgrade pricing is best if you renew before support expires. You can read the Fortran release notes to see what is new. Also note that the 2017 product (with 17.0 compiler) is in beta now and is coming out later this year. You could join the public beta, install that compiler, and see what performance differences there are.
