There's a lot of good sub-compact, single stack 9's on the market. What distinguishes this Sig from the others is a 10 round mag in a grip the size of the others with 6 or 7 rounds. And yes, a extended 12 round is available.
They describe the mag as neither single or double stack and narrowing at the top. They have 3 patents on it.
Like I said, I've shied away from the single stacks because of their capacity compared to my G26 for belt carry. For me smaller is always better for carry. I'll give it a year to see how this Sig goes. My G26 is doing just fine for now.