I have the XD subcompact in 9mm and my pinkie hangs down under the grip. While I do not have a subcompact Glock, I believe the grip length is comparable and my pinkie would hang down under the grip with the 26 or 27. I do manage to adjust and shoot the subcompact just fine even with the short grip.
You may want to think about a Glock 23 or XD service model rather that a grip extension if you want the longer grip. I don't think the length of the slide will make as much difference for CCW as the grip length, so it may be something to think about if you want a longer grip anyway.
Some people like the XD for CCW because of the grip safety. Whether that's a pro or con for you is your decision.
The finger grooves on the Glock are a problem for some people because they don't fit their hands comfortably.
For me, the trigger on my Glock 19 is better than the trigger on the XD.
Some people have problems with the grip angles and like one over the other. I haven't had a problem with either.
I don't think you'll go wrong with either. Try both if you can.
I would also make sure to get a decent holster that fully covers the trigger regardless of whether I chose the Glock or XD.
Today, I would buy the Glock.
