result_of was introduced in Boost, and then included in TR1, and finally in C++0x. Therefore result_of has an advantage that is backward-compatible (with a suitable library).

decltype is an entirely new thing in C++0x, does not restrict only to return type of a function, and is a language feature.

Anyway, on gcc 4.5, result_of is implemented in terms of decltype:

template<typename _Signature>
    class result_of;

  template<typename _Functor, typename... _ArgTypes>
    struct result_of<_Functor(_ArgTypes...)>
        decltype( std::declval<_Functor>()(std::declval<_ArgTypes>()...) )