Some people don't like "plastic" guns. I have a GLOCK and I love it. It's really just personal preference. There's nothing wrong with either. GLOCK pistols are highly reliable and are carried by many LE departments. That should tell you something.
I believe the XD has a grip safety (and maybe a manual safety as well?). GLOCKs have no external safety other than the trigger safety. That is a deciding factor for many people. Also, the grip angle on a GLOCK is much more aggressive than on most other pistols. You have to angle your wrist a bit steeper. That of course is also personal preference.
That's just what came to mind - hope it helps a bit.