C: language interop/FFI
C++: systems programming
But if you HAD to pick one, it has to be C for ubiquity.
Now that I have many many years of experience, would I code more in C++? Lol no, you couldnt pay me enough $ to code in C or C++.
C: Pre C++, C was it. (It was Borland's excellent Turbo C that got me going in C.) After C++ became available C was still what one used for device drivers and other system-level modules, and was the choice for FFI interfacing for languages such as Python.
C++: for userland layers of instrument control systems, user programs, GUIs, etc.
C++ offers a few very important advantages:
* Its object system allows for better abstraction facilities. For example, smart pointers were very useful in mitigating memory leaks. (I even wrote a smart pointer that--under a generalized pointer interface--specialized in allocating and managing DOS extended memory. Remember that shit? Doing the same in C was a PITA.)
* Later, template metaprogramming took this to a new level. Though C++ metaprogramming is not nearly as flexible as the ones in Lisps, it is still tremendously useful.
* The STL arose from both of these features, and provides a rich set of abstractions: queues, maps, trees, etc.
People did tend to overuse the OOP facilities (everything is an object!) but it's hard to overstate how useful OOP and metaprogramming can be. Use C++ unless there is a good reason to use C.