I have a 250, and considered a 320. The 250 and 320 share alot of parts. My 250 is crazy accurate once you get the trigger down. Also very reliable and I don't think there's a question of reliability of the 320.
The drawbacks are the huge slide. Same with the 320. I don't carry it. Also, as far as triggers for a striker on the 320, I don't like it.
So in essence, for slide size and bore axis, as well as trigger, I looked elsewhere.
Considered Walther, much better trigger, same big slide.
S&W m&p and CZ P10 were next. Slightly more trim and slim, better triggers.
I went with the CZ P10.
As far as aftermarket though, the SIG 320 is really only second to Glocks. Then maybe M&P as third.
So in summation:
320 pros are accuracy, reliability, aftermarket with cons trigger and bore axis and bulk.
If you're open to other brands but still want a "mainstream" pistol, I would compare it to the M&P, CZ P10, Walther PPQ/PDP, and Gen5 Glocks. That's kinda what I did