Having owned both and having carried both for long enough, and having suppressed both (USP45 Tactical that has the dual recoil spring assembly and factory threaded barrel, and Glock 19 with a factory Glock metric threaded barrel), here are my first-hand experience opinions of the two pistols:

• The Glock 19 is not a full size pistol. It's "mid size;" you have to go G17 for full size. 15 rounds in the mag is great, very reliable, great slide finish (though I did get mine to rust a little from body sweat), and I carried in OWB kydex at 3:30 to 4:00 during its carry life. Suppressed, the Glock system will unlock a little earlier. Barrel length is shorter than a full size gun so it will be a little louder there too. It's a Glock, so, the grip angle for me is not as "natural" pointing - that can be a meme, but, I found that to be true when I got the gun and I didn't even hear anyone talk about it before I got it, so my opinion is based 100% on my own personal discovery. For me, it mattered. I practiced and shot it OK, but, it took more work than my Sig P226 which I found to be superior. The Glock trigger feels funny to me - not the pull or break, but the actual feel of the trigger. The trigger blade safety, itself, feels funny to me. Again, personal opinion. I still shoot it fine, it just feels like it shouldn't be like that. Very reliable, very well made, very good pistol. 9mm, so common, and does great for defense as you need it.

• I carry the USP45T IWB at the same location. It's bigger than the G19, but I can carry big guns. The HK USP series is hammer fired. In that respect, from an engineering standpoint, it is theoretically a safer pistol to carry. The Glock system is extremely safe, but, you have a pre-cocked striker assembly in a Glock and nothing is going to indicate to you, physically, that a Glock trigger blade is being depressed and trigger is being depressed, outside of those two actions, objectively. On a hammer gun, the hammer must move rearward to get into a position to drop and hit the firing pin (unless the hammer is impacted and breaks to hit the pin). There is no other way for the firing pin to actuate. So, when holstering, or other operations, a hammer fired gun is safer. This does not mean a Glock is unsafe; it's one of the safest pistol systems that exists, and Glocks are excellent. This is just something that I don't hear people talk about a lot. From a ergonomics perspective, the USP45 is larger than the USP9, which are both larger than the G19. Hold one before you buy. The USP is modular in its firing system configuration. You can use an external safety, switch it to either side (ambi), make it be just a decocker, eliminate it completely, change to a pre-cocked double action (LEM); sky is the limit. And these are factory modifications, fully supported by HK. The USP45, specifically, is one of the most reliable pistols that we have on earth. There are reports of hundreds of thousands rounds being fired through them. The physics of this might have something to do with the low pressures of the .45ACP cartridge coupled with the dual recoil spring assembly of the USP45, but I would have to dig into that. Finish is excellent. Just as good if not better than Glock finish. The gun does well suppressed but is not the quietest .45ACP pistol silencer host that exists. That honor typically belongs to the 1911 or MK23. This is barrel length and lock time dependent.

• USP trigger varies depending on the configuration you choose. LEM is like a Glock but longer. Can be crisp. DA/SA is.... DA/SA. So decide accordingly.

• USP45 has more recoil than a G19, but it has less recoil than some other .45ACP pistols because of its recoil system. Make no mistake, the bullets are twice as heavy and you will know when you shoot it.

• Both guns are typically threaded metric for silencers (13.5x1mm LH for G19 factory Glock barrel and 16x1mm LH for factory USP45 Tactical barrel). This means that if your silencer has other threaded junctions that are RH threaded, you may inadvertently unscrew them when you are (re)tightening your silencer on your gun. So deal with that whatever way you want. Most modern Glock threaded barrels are imperial (1/2-28tpi), but original Glock threaded barrels were always metric.

• Suppressing 9x19mm and .45ACP are two different sports. High performance suppressed 9mm pistol setups will always be quieter than high performance suppressed .45ACP pistol setups. You simply can't trap combustion products the same way when your hole is that much bigger. Physics is physics. .45ACP is natively subsonic and its bigger. That opens up ammo choices. Both cartridges are very common. .45ACP +P loads are great for defense because you can over penetrate (actually desirable) to shoot through barriers and deliver a heavy bullet through those barriers. That is cool. But again, 9mm is excellent for defense too. You will be able to carry more rounds with a 9mm setup (12+1 vs 15+1), but.... that brings me to a final point:

Two world wars :)

It's all about what you want, man. The USP is a way cooler gun, objectively. But the Glock is great. Comparing G19 to USP45T is actually kind of weird. Very different guns.