In my opinion, it all depends from the model you want to realize.

If your model can be just expressed in matrix terms, then nothing beats fortran efficency and expressivity.

In all other cases C++ is the best choice.

Just to give a more concrete example: if I want to model a soft computing technique like, for example, a neuro-fuzzy engine to process certain data, I would surely use C++. Would I need to realize a code for a fluid dynamics simulator using pseudospectral methods, I would surely use Fortran.

Best thing is that, as already underlined, using F2003 ISO_C modules you have full interoperability between languages, so it's common for me to call C++ routines from Fortran programs and vice versa (just remember to declare C++ "entrance" points as "extern "C"" since ISO_C is designed to manage interoperability with C) without sacrifying portability.