I'm sure you mean the P220ST stainless, right?
I own a P220 and P220ST and have not owned an HK USP45, but have fired them quite a bit. And I actually like the USP also, but only second to my Sig, and cannot compare them to the P220. I had a tough choice of having to choose one and ended up loving the P220 over the USP. I once again had an opportunity to purchase a USP45 but in stainless. But then this darn P220ST came out and bought that instead. Glad I did with NO regrets! The simplicity of the Sig is great. And easy take down in 4 seconds or less. No need for manual safety cause Sig uses a patented firing pin block along with its 4 point safety. Almost impossible to accidental firing unless trigger is purposely pulled. For CCW, also go Sig. And plstic verses steel or aluminum. Unfortunately in my area, LEO's have to have a manual safety on their guns, so most want to carry a Sig but can't as a primary duty weapon. But a local Police branch has all turned to Sig Pros. But I am not too fond of the Sig Pros.
But Hk and Sig is a great choice and you will not regret either. Been in your shoes twice having to choose b/ these two and I am happy with my choices by going with SIG! But if I buy another .45, I am sure that it will be an Hk USP.