No offence, but I think you're asking the wrong questions.
It's not that things like availability of extra mags, holsters, and spares aren't factors, because they are, but things like build quality, hand feel, and the trigger action are (in my opinion) so much more important.
I just went through a really long research process comparing all of the guns in the category, and bottom-line-up-front: I chose a Walther PPQ M2 Navy with the 4.6" threaded barrel. When I have the barrel shaved down to 106mm it will look like this:
When I started the process, the Walther wasn't even on my radar. I went to Epps and said I wanted them to bring out everything like a Sig, H&K, Glock, M&P, SR9, etc, to compare. After listening to what I liked and disliked about all of them, the guy asked "Have you ever thought about a Walther?", to which I replied "Not really", but then he brought one out. Everything about the PPQ was awesome; the "wow factor" build quality was on par with H&K and Sig, but the hand fit was incredible. And then there's the trigger, which is absolutely amazing, but don't just take my word for it, go research all the comparisons online that review it as having the best trigger in the category.
I as not a fan of the trigger guard paddle mag release, though, but after doing so more research I found out about the M2 model with the conventional button-style mag release. Epps couldn't get the M2, though, but IRG had one on sale, and I bought it. Have had it out shooting and 100% stoppage free, which even my Glock couldn't say (although I mark the mags on the bottom after a stoppage, and it was the same mag every time).
Back to the main point, I think it's more important to focus on getting a good gun than worrying about accessories. Sure they may be harder to find than Glock accessories, but you're getting a gun that does everything better than everyone else. The only thing I don't like about mine was that it came with a serial number that started with "FAT".