I would recommend C++ if your platform supports it (and if you choose to go C/C++), because it's higher level than C, and higher level always means making it easier for you. As well, it is a somewhat modern language with powerful features and tools - but the best thing is that in case you need to go lower, then C++ is backwards compatible with C and hence you can do those lower parts in C, too. Or you can compile them using a C compiler, if you wish.
And let's be fair about one thing - the myth that C is faster than C++ just is not true. They have advantages and disadvantages and are thus about equal in speed.
C and C++ can sometimes also deal with assembly, although it's tricky and requires safety guidelines.