there are many things that separate the regular and the XDm from each other.
I have shot both and found that the XDm lends itself more towards fine tuning for the shooter. I personally have a hard time finding a REAL difference between the two that my skills as a shooter can exploit. I shoot both to the same Qual score consistently and just couldnt justify the difference in price for the XDm. Thats just me though.
Looking at the difference in the springs makes me think the recoil would be different but i totally forgot to pay attention when i shot the XDM-40. Obviously the capacity is remarkable but the carry isnt perfect. I shot the same too. For me the trigger on both is perfect for double action. I see no reason to buy just one or the other, both is the way to go. XD RULES!!!!
Ive had my XD subcompact 9mm for about 3 years and my XDM 9mm a few months and I like to shoot the XDM when at the range more but I like the sub for a carry pistol. The XDM just seems to shoot better,for me. Ive always thought the XD sub wasnt very accurate when taking it to the range with my other 9mm pistols- a Beretta,a Glock,a Ruger,an M&P,etcetc. But then I have never done an intensive study of the difference in the 2 pistols and why the XDM shoots better. I'm not a competitive shooter or a pro either,just a plinker,so my experience with the two may not mean anything to any body.
Handle them both and think about the intended purpose. I had an xdm in .40 cal. and liked it far better than the xd. And yes I did shoot many xd's in 9mm, .45acp, and .40cal. For me the xdm was better. I wouldn't want to carry it on a daily basis though. It is on the verge of a full size gun, 4.5'' barrel.