I do not own an XDM but I am a big fan of the XD45 as I own two of them and carry 1 on duty.
XD 45 is a soft shooter, accurate and works well for those with average sized hands. 13 rd capacity.
Negative of it is that you ave to get the .45 ACP ammo.
XDM is said to be an improved model. High capacity,16+1 IIRC.
Trigger is said to be improved, pimping reduced reset etc. I think the .40 looks pretty sharp. If I did not have a preference for caliber and I just wanted a new blaster, I would get the XDM over the XD45.
That being said, I like the .45 and I am downsizing my .40 collection so I bought my pair of XD45's over an XDM 40 recently.
I traded my XD-40 so I could buy an XDm-9 ... it was a great move ... the M is a big step ahead of the XD; fits better, shoots better and holds a hell of a lot more ammunition ... I like .45s, shoot two 1911s ... but the M is a better deal; get the .40.
Honestly, ballistically speaking there isn't a big difference between the two calibers. I would choose the gun that is most comfortable to you. For me at least, the XDM feels a heck of a lot nicer than the regular XD.