I have all three (sans the Tactical bit) USP45, USP9, and the VP9. I shoot all very well; the USP45 the best with the VP9 and a very close second.
With that said, I have fired more rounds, and continue to fire more rounds out of my USP9 than the VP9. I love the VP9 and it is a pistol that isn't going anywhere. I'd be splitting hairs to say that I shoot one of the other better because it comes down on what side of the bed I woke up on. I will say I do not pull to the left hardly at all with the VP9 as I do with the USP9 and that is because of trigger. I am also faster with the VP9 on follow shots. The VP9's trigger is the key. I have fired pretty much all variants of pistols and understand both striker and DA/SA. This is coming from a guy who grew up on 1911s as a kid, so I was spoiled on a good trigger.
Shooting wise and price point, the VP9 is hands down the best!
But end of the world times; or just to get me home from a bad disaster (OTR trucker and I carry), I take the USP9 for its size, capacity and caliber. None the least, reliability. (I've fired 7,000+ rounds through mine for the 3 years I had it).
The USP9 will get you better with your USP45T. In fact, you will shoot it a lot more due to the cheapness of the ammo and mags. USP9 mags are 1/2 the price of USP45 mags. The are very durable and hold up a bit more than even Glock mags. The Jet funnel mags will fit the USP9 without the Jet funnel kit (I'd get the steel ones if you are going to keep them loaded over time). I'd also get the 10 pound hammer spring from Wolff. Makes the double action about as smooth as the USP45.
The USP9 will be smaller than the USP45T, and you will have to order the extended base plates for the magazines if you are used to the USP45 mags. I prefer the grip of my USP45 as it seems to fill my hands better while I fill I have to adjust my grip on the USP9 at times (about to get grip tape for it shortly). It does have the high bore axis but without out the O-ring (unless you get the tactical or expert). It shoots very smooth, however.
So, if you are wanting a shooter that is fast and accurate with a good crisp trigger that seemed to to be lacking in almost all HKs (sans the P7) and save some coin for ammo, Get the VP9.
If you are wanting your "grail" and want to have a very close clone to your USP45T to save wear and tear (like that exists on an HK) and can get mags for it cheaply and right-damn-now, get the USP9. Probably by the time you are in the market for the VP9, not only can you get it, but mags should be back in supply.
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