It depends on what you are planning on doing with the XDm. If you are going to shoot competition. And need the extra capacity of the XDm magazines. Then the XDm might be worth the extra price.
But for normal shooting (IMHO) the difference between the XD or a Glock and the XDm is not worth the extra price.
Having said that. I bought the XDm in 9mm anyway. Just because I like the looks of the pistol and I am a Springfield XD fan. So I had to have the XDm.
If you are just looking for a good gun and are trying to choose between the two. The Glock is every bit as good as the XDm for normal use.
I would suggest renting both guns if you are not already familiar with them and get some trigger time with each. After shooting them you may decide that you like the XDm and that it is worth the extra cost.