Like others, I own both, like both, and trust both. But I shoot the XD just a bit better--let me rephrase that: it aims more instinctively for me--than the Glock. I owned 1911s long before I bought my first Glock. I don't really have any problem shifting from one platform to the other, but if I were to close my eyes and have you hand me each in turn, I could keep my eyes closed and point the XD with reasonable accuracy.
And comparing my XDm (.40) to my G22 and G27, the trigger is definitely better on the XDm: much crisper; not as mushy. The stock XD also has a decent trigger, but it has an awfully long, though positive, reset. My unmodified G35 has a trigger action that's somewhere between the G22 and the XDm. I have an XD9 tactical that was worked over by Canyon Creek Custom, and can confirm that the trigger break and reset length can still be improved upon on the stock XDm. (Canyon Creek also sells aftermarket spring sets and such for the XD.)
One tiny thing that I never see mentioned is that the two, slightly different striker activating mechanisms make a difference in dry-fire practice. With the Glock, once you release the striker, you essentially have a depressed trigger with no movement until you cock the striker again. With the XD, the spring returns the trigger to its original position. The result is that you can do a level of dry-fire practice with the XD without having to cock the striker. Admittedly, the pull-weight isn't the same, and you get no tactile feedback of when the shot would break, but you do get to move you index finger against spring tension throughout the range of motion.
To me, that's a big plus for the XD. One of the downsides of dry-fire is that you can't practice more than one shot at a time with many pistols. To practice realistically, you need to be able to draw, move, and shoot mutliple times, and the XD allows you to simulate that without having to actuate the striker. (And I agree with Gabe Suarez that that majority of practice should be done without live fire.)
As cajunautoxer mentioned, with so many Glocks in the world for so long, the parts aftermarket is huge. You can easily find anything. But XD parts are available; certainly all the basic parts you'd need for maintenance. Brownells' carries trigger kits, spring sets, guide rods, strikers, and what not. And other places like
PistolGear.com sells both replacement and aftermarket "improvement" parts. So not nearly as plentiful as Glock parts, but they're out there.
Some experienced gunmen I know have shied away from the XD because they were familiar with its Croatian predecessor. Since the XD has been on the market now for several years, my suspicion is that the quality control and manufacturing consistency that Springfield brought to the table has started to allay some of those concerns.
The new Gen2 XDm-3.8 looks interesting, but I don't mind the 4.5" barrel . But I'll tell ya what I keep watching for: an XDm in .45 ACP.
![Wink ;-)](./images/smilies/icon_wink.gif)
I might even part with my XD .45 Service if an "m" version comes out...