As far as dimensions go a Glock and an XD are virtually identical as far as slide dimension goes (both just over an inch) so that arguement isn't really accurate. For a true CC gun you can go with the SC which has a shorter grio than the the 4" models and the grip is the main part of the gun you have to conceal. Some have no problem with longer grips but the SC is shorter and therefore easier to hide.
If you are just looking for a range gun (not CC) the choice between the 4" and 5" models is really a matter of preference. Overall you'll find that the XD has excellent feel and natural pointability, is very accurate, is damned reliable with most any ammo you want to shoot (and is okay to shoot lead which one should not do in a Glock due to the polygonal rifling) and the night sights are excellent. Not to mention that the trigger is excellent and can even be improved with a few bucks and a trip back to SA.
If you want to discuss or see further details about an XD you should check out their forum- there are a bunch of good, knowledgeable guys (and gals) there that share a lot of info and ideas : http://www.xdtalk.com/forums/
"Shoot low boys...they're riding shetland ponies in search of true grit" Lewis Grizzard