Mutation testing is widely used in research for evaluating the effectiveness of test suites. There are multiple mutation tools that perform mutation at different levels, including traditional mutation testing at the level of source code (SRC) and more recent mutation testing at the level of compiler intermediate representation (IR). This paper presents an extensive comparison of mutation testing at the SRC and IR levels, specifically at the C programming language and the LLVM compiler IR levels. We use a mutation testing tool called SRCIROR that implements conceptually the same mutation operators at both levels. We also employ automated techniques to account for equivalent and duplicated mutants, and to determine minimal and surface mutants. We carry out our study on 15 programs from the Coreutils library. Overall, we find mutation testing to be better at the SRC level: the SRC level produces much fewer mutants and is thus less expensive, but the SRC level still generates a similar number of minimal and surface mutants, and the mutation scores at both levels are very closely correlated. We also perform a case study on the Space program to evaluate which level's mutation score correlates better with the actual fault-detection capability of test suites sampled from Space's test pool. We find the mutation score at both levels to not be very correlated with the actual fault-detection capability of test suites.