You have plenty of options to interface C and Python. You can use Cython to create Python bindings to C libraries or even better you can use cffi which works with PyPy as well
I'd rather work directly in a full-featured, internally-self-consistent language (and Nim is a very nice language) from the start. Hence -- for me, at least: Nim. :)
Aside from an entirely-subjective preference that caused me to fall in love with Nim at first sight, there are practical reasons for choosing Nim over Julia.
The overarching reason is that there is much more Python code in the world than Nim or Julia put together, and I think it will remain this way for a long time. (My startup previously worked almost completely in Python, for example, aside from some C or C++ extension modules and the usual Bash scripts for sysadmin.) Python also has some solid webserver frameworks, and is great for a variety of general-purpose tasks.
So, I wanted a solution that would enable us to incrementally rewrite code as necessary, without needing to rewrite everything from Python all at once. Also, I have no desire to rewrite our webserver code from Python. Flask & Django are just fine.
As I understand it (and please correct me if I'm wrong), Julia doesn't compile to C. So I can't compile snippets of Julia code and call them from my Python main-loop.
In contrast, not only is this possible in Nim, but Nim's macro system & well-developed AST spec make it very easy to traverse my Nim procs during compilation, to auto-generate per-proc wrapper code in C.
Cheers!
Cython is certainly great, but still Python objects and C structs differ, and you have to keep in mind the difference. Same for C functions and Python functions. And if you use the dynamic features of Python, you do not gain much in efficiency, because Cython does not have static dispatch, hence it has to rely on runtime checks.
In a way, Cython is its own language, and it is not a particularly powerful one. The help you get from types is limited, as many things will just have type object. At this point, I just prefer to use Nim, which is friendly enough, and has overloading, generics, macros and many other features.