Having the opportunity to handle and test-fire an example of each might go a long way to answering many questions ...
Which us a better truck? Ford or Chevy? Toyota or Nissan?
When you ask such questions on an internet firearms enthusiasts forum you're going to get a lot of opinions and personal experiences.
Do you want a plastic or metal frame?
Do you want a single column or double column magazine?
Do you have a preference for the trigger mode?
Can you shoot either of them well?
Both are good quality examples of their respective company product lines when it comes to service-type pistols.
I've been to the Sig pistol armorer class, but not the HK USP pistol armorer class. I only know a couple of HK USP armorers. Parts and support is not exactly high on their list of favorite topics of conversation.
The 220's with the external pivoting, plunger & spring tensioned extractors are the newest ones.
As a long time owner and shooter of various .45's, and having shot examples of each design mentioned since they were introduced (which means back when the P220 was being imported as the Browning Double Action in the case of the Sig) ... I haven't felt the inclination to spend my own money on either of them. I prefer other makes/models ... but that's
me.
Try to find a way to rent and test-fire each and see if either interests you.
There are going to be satisfied and unsatisfied owners of each brand to be found on the internet. Hardly surprising.
I do own 9 pistols chambered in .45 ACP at the present. They include Colt, Ruger and S&W (both 3rd gen and M&P pistol). I anticipate my next .45 purchase is going to be another M&P, this time a compact (M&P 45c), and maybe followed by a Performance Center SW1911.
Like I
need 11 .45's ...