Long time XD fan/owner who just got my first GLOCK for a variety of reasons. So let me share my opinion on the two.
To be fair the 19 is a compact where as the XD is not, so keep that in mind, but the XD doesn't offer a mid size/compact in 9mm at the moment. I wish they did.
1. Size - goes to the 19. It is considerably smaller both in muzzle length and grip length without sacrificing much in the way of versatility or usability.
2. Capacity - goes to the XD. 17 rnds vs. 15 rnds. However keep in mind the 19 has a shorter grip (better for carry), so to me is a good trade off. If I really need those extra 2 rnds, I'm in it deep anyway and more mags are propably more important anyway.
3. Trigger - i know it's subjective, but I go with the XD. With both having trigger adjustments: 19 Ghost 3.5 lb. connector - XD Powder River Safety lever - the XD's reset and break are much better IMO. The 19 is not "bad" by any means just different and the XD's is more like a traditional single action trigger to me.
4. Safeties - XD. I "feel" a lot more comfortable with the XD's numerous safeties. The visible striker, loaded indicator, and grip safety are great additions to me. While the 19 has similar features (excluding grip safety and visible striker), the XD's are more positive and clear.
5. Weight - Glock hands down. Don't know actual weight, I'm not that anal, but using "feel" and manufactures numbers - the 19 wins easily.
Conclusion? The 19 fit my carry needs better than the fullsize XD, but I like the XD's features better. Not so much that it is a big deal, but given my druthers I'd like a Glock/XD hybrid! I also find the availabilty of parts and accessories, which also are some what cheaper, is a plus as well.
I haven't got a chance to range test the 19 like i have the XD, but I will let you know when I do. these are just my initial impressions.