A simple grep of the SDK shows us that the answer is that it doesn’t matter—they are the same. They both turn into __T(x).
C:\...\Visual Studio 8\VC>findstr /spin /c:"#define _T(" *.h
crt\src\tchar.h:2439:#define _T(x) __T(x)
include\tchar.h:2390:#define _T(x) __T(x)
C:\...\Visual Studio 8\VC>findstr /spin /c:"#define _TEXT(" *.h
crt\src\tchar.h:2440:#define _TEXT(x) __T(x)
include\tchar.h:2391:#define _TEXT(x) __T(x)
And for completeness:
C:\...\Visual Studio 8\VC>findstr /spin /c:"#define __T(" *.h
crt\src\tchar.h:210:#define __T(x) L ## x
crt\src\tchar.h:889:#define __T(x) x
include\tchar.h:210:#define __T(x) L ## x
include\tchar.h:858:#define __T(x) x
However, technically, for C++ you should be using TEXT() instead of _TEXT(), but it (eventually) expands to the same thing too.