If this is a first handgun purchase, I'd would heartily advise getting the 9mm over the .40S&W. You'll get better capacity and less felt recoil, though this isn't exactly true for some. 9mm does seem to me to have a bit quicker recoil impulse with less overall imparted energy, so it seems a bit "snappier". The real benefit is that 9mm is cheaper, which allows you to shoot more and develop more skill. True skill wins over stopping power any day. That, and the stopping power issue is subject to debate. I wouldn't use 9mm with hardball ammo if I could avoid it, but with modern hollow-points, I'd feel more than comfortable with it.
One can't really assess the true difference between quality and reliability given that the XD has only been around for a short period of time and the Glock has an extensive service record. It should be enough to say that the XD has sufficient quality and reliability and the Glock has an excellent record.
Grip angle and shape seem to be the big difference. The XD has a grip angle similar to a 1911 while the Glock has a steeper angle requiring more cant in the wrists. It is my personal opinion that the steeper angle fosters a better grip, but then that's just opinion. The depth of the grip on the XD seems to be shorter front-to-back which might accomodate smaller hands better. One should shoot both to figure out which is more comfortable in the hands. However, I'm not sure how valuable this would be to an inexperienced shooter that hasn't developed a proper grip.
I'll offer my opinions with the caveat that I prefer and shoot a Glock. I have shot an XD quite a bit and don't really like it as much. I believe that the lower bore-axis of the Glock can make recoil management a bit better with proper grip and a little added relief on the trigger-guard.
As stated, the XD usually has a better trigger out of the box. However, there are many different permutations of the Glock trigger than can affect one's perception. There are a few different connectors that are available as well as production variations between pistols. A friend purchased a G23 at the same time that I purchased a G35. I threw a 5Lb connector in mine to compare them side-by-side and discovered that his trigger was a bit nicer and crisper from the factory. I examined the parts in either and couldn't come up with a solid explanation for the difference. The parts were mostly identical. In any event, both the Glock and the XD could both use some trigger work as they come from the factory. Yes, this even holds true for pistols intended for carry.
The factory sights are better on the XD, but that's usually the first item to get replaced on a Glock anyway. Of course, this increases the price differential, but imho, a Glock isn't complete without a set of Heinies.
One of the things that's probably more important than deciding on which brand to purchase will be deciding on which length to purchase. For the purposes of shooting comfort, I would stay away from the compact and sub-compact versions unless they are specifically intended to be carried concealed frequently. A G17 is more pleasant to shoot than a G19. A G34 may be more pleasant to shoot than a G17, but opinions vary on this. A few prefer the felt recoil of the G17. XD has similar length models in either caliber.
Magazines for the Glock are plastic and metal-lined. I've never had a problem with a new magazine dropping free. However, they can develop issues if they get scratched or scuffed. Older magazines of earlier generations can also have issues. XD magazines are certainly prettier, but they're also less common and a bit more expensive.
Good luck with your decision. Either is a good gun, so you should be fine in any case. If you get one and decide you like the other, you can always sell it and then get the other. Then there's the best possible solution, which is to get both.
