I own both, and they are very different guns for different uses.
The TP9 is not a pistol, it is in the same ballpark as an MP5K or Mini Uzi. Which means it is on the small side of what most people consider to be a carbine. Without the folding stock and forward grip the TP9 is football bat of a pistol. It is huge, unwieldy and over-complicated. But as a carbine it is a nice, tight package that has benefits above the MP5K PDW... however for every benefit it has, it has a drawback. The biggest drawback being its incompatibility with pretty much every pistol cal suppressor on the market due to its unique rotating barrel action that necessitates its oversize bell housing of a muzzle. Your choice in suppressors is the B&T factory can... or nothing. That said, as an SBR carbine with the correct (or as correct as I could get on my first gen) it is a very ergonomic and fun thing.
The USW on the other side of the fence is very much a pistol. It is an oddly shaped pistol with additional provisions to have a folding stock mounted to it. And the "folding stock" should not really be seen as much of a stock. Due to its short LOP and undersized butt, the USW's folding stock is more like a folding support that you jam into your shoulder to allow you greater stability when aiming. The gun does not really lend itself to forward grips, you are pretty much stuck with it as is. Which means you are either shooting it as a large roland special style handgun, or you are scrunched up shooting it as a teeny tiny SBR. But much like the TP9 for every positive it has, it has a negative. Yes with the traditionally threaded barrel you can use any common suppressor, but due to its odd shape and accouterments it needs a special holster.
If you forced me to choose one and only one, I'd have to go with the TP9. It is not a decent pistol but it is an excellent 9mm PCC that checks all the boxes a 9mm sub gun clone needs to check. Small, light weight, good ergonomics, good magazines, top rail for diverse optic choices, pretty ammo friendly, and not entirely obnoxious to actually shoot. The big reasons I am not absolutely in love with my USW is because it is a jack of all trades and a master of none. It is not that great of a service pistol due to all the adjustments made to it to allow it to have a stock, it is not that great of an SBR because it is so tiny and its manual of arms is very awkward. What it does do is allow you to carry a PCC SBR in a holster on your hip... even though you can get a similar holster for the TP9, so that selling point is kind of moot. Well, the USW is smaller.
My advice would be avoid the USW, buy a TP9, then if you are so inclined buy the USW stock adapter kit they are making for another existing gun and get 80% of the USW experience for 40% of the asking price.