what is more effective: std::strings or numbers?

what is more effective: std::strings or numbers?

i got a collection of keywords. they are all up to 12 characters, consist of
only lower case english letters, and we may expect, that no new string will
be longer in the collection. we can convert such strings to numbers and
identically recover them, say, this way (ms specific):

#include <string>
using namespace std;

const __int64 Str2Num(const string &S)
{
 if(S.length() > 12)
  return 0;

 __int64 n = 0;
 for(size_t i = 0; i < S.length(); ++i)
  n = n*26+__int64(S[i]-'a'+1);
 return n;
}

const string Num2Str(const __int64 N)
{
 if(N == 0)
  return "";

 string s;
 __int64 n = N;
 while(n >=26) {
  s = char((n%26)+'a'-1)+s;
  n /= 26;
 }
 return char(n+'a'-1)+s;
}

now i want to put my values to an std::map. what will be more effective:
std::map<__int64, something> or std::map<string, something>? __int64 takes 8
bytes, string takes more - 16. but which comparison will be faster - strings
or __int64.

does this converting make sense?

thank you, guys.

ps. ms vc++2005.

--
alex c.