Какая реализация лучше? std::pointer_to_binary_function vs std::function

#include <iostream>
#include <vector>
#include <set>
 
using std::string;
using set_p = std::multiset<string, std::pointer_to_binary_function<const string&, const string&, bool>>;
using set_f = std::multiset<string, std::function<bool(const string&, const string&)>>;
 
bool len(const string& x, const string& y)
{
   return x.size() < y.size();
}
 
int main()
{
   std::vector<string> words{"freedom", "tree", "fox", "green", "less"};
 
   set_p set1(words.begin(), words.end(), std::ptr_fun(len));   
   set_f set2(words.begin(), words.end(), len);
 
   //for (auto& i : set1) std::cout << i << "\n";
   //for (auto& i : set2) std::cout << i << "\n";
 
   return 0;
}

using set_f = std::multiset<string, decltype(len)>;

using set_f = std::multiset<string, bool (*)(const string&, const string&)>;

#include <iostream>
#include <string>
#include <vector>
#include <set>
 
using std::string;
 
auto func = [](const string& x, const string& y)
{
   return x.size() < y.size();
};
 
using my_set = std::multiset<string, decltype(func)>;
 
int main()
{
   std::vector<string> words{"freedom", "tree", "fox", "green", "less"};
   my_set set(words.begin(), words.end(), func);
 
   for (auto& i : set) std::cout << i << "\n";
 
   return 0;
}