Group size. I bought a 3.8" XDM in 9MM a few months later and everyone that shoots both shoots the 3.8" better. Not just me. Everybody likes the "little gun". Maybe the 5.25 is one of those guns that needs a "special" load for it. Some guns seem to not shoot much of anything very well unless you find that special load. My M1A is like that.
I agree I cant get the accuracy out of my XDM that I would like to, especially compared to my P-09, but dont think its terrible. I have several different loads for my P-09 that will group 1" at 25 yds, the best group I ever shot with my XDM was 2" at 25 yds but 3" groups are far more normal. That said, the XDM works better for me in steel challenge matches and IDPA, and my P-09 works better for me in Action Pistol/Bianchi cup matches, and they are somewhat equal in USPSA matches. Im hoping the P-10 can take the place of the XDM and give me the accuracy of the P-09, and its always fun to get new guns, right?