I own a XD45 Tactical... considered upgrading to the M.
From what I understand the trigger pull and reset are shorter on the M and the barrel and sites are better than the standard XD as well. Throw in the interchangeable backstrap and you really can't go wrong.
If you have the choice, I would go with the M. But that's just me...
I personally love my XD, not in a hurry to upgrade. But if I had the choice to make again now it would be for the M