vstudio, debug, release and assembler

[monarch_dodra]

I've been using vstudio's debug and goto assembler functions recently, and I had a few questions:

For starters, does it make any sense to debug a release build? If I understand correctly, the debug symbols may slow down the program, but the final assembly should be the same?

I was trying to see if I was better of using 2 iterators to "add two vectors", or just use 1 shared index. This is my test code:

#include <iostream>
#include <vector>

int main()
{
  system("pause");
  std::vector<int> a(10000);
  std::vector<int> b(10000);
  for (size_t i = 0; i!=10000; ++i)
  {
    a[i]+=b[i];
  }

  int* pa = &a[0];
  int* pb = &b[0];
  int* const pEnd = pa+a.size();
  for (;pa!=pEnd; ++pa, ++pb)
  {
    *pa+=*pb;
  }

  std::vector<int>::iterator ita = a.begin();
  std::vector<int>::iterator itb = b.begin();
  std::vector<int>::iterator const itEnd = a.end();
  for (;ita!=itEnd; ++ita, ++itb)
  {
    *ita+=*itb;
  }
}

Looking at dissambly, VS is terrible at optimizing the iterators: tons of instructions, bunches of function calls...

The same code with pointers was optimized to 5 assembly instructions, of which 1 single jump(!)

I find it strange that in release, I get this order of performance :
pointer > index > iterator

Now I did not time the program, and I don't really want to anyways, the goal here is trying to understand optimizations.

am I doing something fundamentally wrong trying to look at a release build? Does VS have some kind of safe vector iterator even in release?