C, because
- a program written in C++ can use a C library, the other way around is a whole lot trickier - any construct available in C++ but not in C cannot be exported to a C program, which risks to make your export complicated.
- A C library will have less dependencies. C++ libraries will quite often depend on libstdc++, which is that big that for example on embedded systems or more generally on systems low on resources it won't always be available.