If you need
case sensitive comparison you need your used strcmp, but if I strongly recommend that you use functions for the comparison like:
BOOL isRock(const char* choice);
and so on.
Your function should have a better interface
char rpsgame(char *choice, char *choice2)
Additional you can check with
isupper that the first char is an upper char,
BUT any valid choice must be on of the "isXYZ" functions. :-O