They are both; there are things that Rust's macros can do metaprogramming-wise that C++ templates cannot do and vice-versa.
Rust's macros work on a syntactic level, so they are more powerful in that they can work with "normally" invalid code and perform token-to-token transformations (and in the case of proc macros effectively function as compiler extensions/plugins) and less powerful in that they don't have access to semantic information.
