The 12 rounder that came with each of my two 365's are all I need. I have two 15-rounders "just because", but the only magazine I carry is the flush 10-round magazine. I like my small carry guns to be small. I also am happy with the pinky dangling under the magazine. Some hate that, but I find the grip to be firm and comfortable.I am having a hard time deciding if I want the 12 round mag - I like the feel of the 10 round without the finger extension better.
I do wish SIG mags were less expensive (like Glock), but it is a quirk I have lived to deal with (similar to H&K mag costs). Just like Glock's need better sights, and result in an added $50-$100 per gun vs SIG, the mags for the SIG add $50-$100 compared to the cost of a Glock.
In the end, it is similar enough, and it gives me something to grumble about!