Newby C++ vs. C question

Hi guys,

I am a newby in the C/C++ world, and I am beginning to work on a
rather simple TCP/IP proxy application which must be able to handle
large volume of data as quickly as possible.

Since this application has to process and distribute plain text around
the network, I am wondering if there are any peformance differences
between C++ std::string and C char[] in run time?

Which one would you suggest me to use for my particular task (TCP/IP
proxy which is distributing plain text around the nextwork that
is :-) )?

Thanks, Alex

p.s.: here're two examples that I found on the Internet for which I am
wondering if there are any performance differences between them:

==========================
C function returning a copy
==========================
char *fnConvert(int _ii)
{
   char *str = malloc(10); /* Return 10 character string */
   if(str == NULL)
      fprintf(stderr,"Error: Memory allocation failed.\n");
   sprintf(str, "%d", _ii);
   return str;
}

==========================
C++ function returning a copy
==========================
string fnConvert(int _ii)
{
   ostringstream ost;
   ost << _ii;
   return ost.str();
}