Bob,

I also have specific gun holsters for my P95 and P9. My holster size reference was from an online chart which showed the G23 to fit a size smaller holster than the XD-40.

My point in mentioning the holster size was only to show that by at least one measurement, the G23 is slightly smaller than the service model XD-40 (despite both having a 4" barrel) and therefore seemed to be slightly more concealable when carried.

I expect, though, that when carrying a fully loaded mag, the G23's extra round could cause it to weigh close to the same (though the plastic mag of the G23 vs. stainless steel mag of the XD would keep the XD heavier). My personal experience in handling both guns empty is that the XD-40 is slighter heavier (21+ oz. vs. 27+ oz.)...and the G23 with no safeties to catch on clothing, would be somewhat easier to draw. 

-Sam