[LuckyTran]

Just pick one. Or start learning both and see which one you like more. Your task seems to be to write some kind of working solver and not to actually become a guru of C++ or Fortran. It makes sense why they didn't give you any more advice. There is no advice to give because that's not the issue. If you're still learning the language then it won't ever matter how fast either of them execute because it'll always be faster than how fast you can write the code.

Originally Posted by sbaffini

Linux hands down... I can't think of a single library or relevant development tool that is available in Windows and not Linux.

Seconded because most of the C/C++ libraries in windows come from Visual Studio which are just (poorly) ported versions of linux libraries where everything works as intended. Skip this headache and go linux.

Actually a good reason to choose Fortran or C++ is to pick whichever has more libraries you are more comfortable using. As an example, imagine you are writing a code regular C and didn't know what functions are in stdio.h or math.h. Oh boy...