Which model of the Springfield XD pistols gets you the best "bang for your buck"?

I am looking into buying my first pistol and have settled on the Springfield XD model. However, I do not know whether to go with the service model, sub compact, compact, tactical, or the ACP. Also, there is the XDm model, what is the difference between the XD and XDm. And which model is the top of the line?