I have a Glock 19 that carry when I am not here in Iraq. (army) I was thinking about either getting a Glock 23(.40S&W) or a Glock 20(10mm). I am not worried about the concealment at all.
If you reload the 10mm is GTG. It may be an issue finding ammo. If you've shot 10mm and can control the recoil fine, there is no reason to "downgrade" to the .40S&W. If you don't reload finding 10mm may be a problem (and it will most certainly be expensive if you CAN find it)
Why do that? Just download the 10 to .40 specs. That used to be the carry load for the 10mm, before the dirty .40 all but killed it (Only half kidding about the dirty .40 part, I sometimes wonder if my mild dislike for the .40 comes from the fate of the 10mm.) I loved the 10mm, and the versatility it offered. The mid-range load for defensive carry, with the option for full power rounds when that was desirable.
I understand being torn. But either will be -- IMO -- an improvement over the 9mm you're carrying now.
For me, the main factor would be ammo availability. .40 is everywhere in any flavor you want. 10 mm is scarce and limited in loadings. So, unless you're going to load your own, 10 mm is going to be frustrating. And the 10 mm is much more expensive, too.
I recently got a 10, but it's for fun, not for carry. I carry a 40 all the time.
A model 20SF or a model 23 , both are great guns , you won't regret either , but the 10MM is my favorite , ammo is scarce but it's out there , you just have to stock up when you see it.
10mm is a reloader's dream caliber. Could be a bit of a hassle for a non-reloader though, as ammo isn't always available, or not in a great selection, and/or not at reasonable prices.
JUST MY PERSONAL OPINION but I don't think there is going to be a huge difference in real world self defense effectiveness between these two calibers. If you are hunting wild boars that weigh three quarters of a ton and have bone structure 3x more dense than human, then yes 10mm. If you are just going to stick a gun in your holster for self defense against people, either will be fine.
Never shot and don't own a 10mm but would like to on both. I have a couple .40's and love the round. Not that you asked this but I carry a .9mm because it holds a couple more and the recoil is less. That helps me be better with my accuracy in a bad situation.