I'm going through this same thing right now. I really want a 45 and I'm a 1911 guy, but the little ones just cost too much. So the Shield would be a nice 9 carry- I like my full size M&P 9, but I still want 45. I had narrowed it down to the XDS, Glock G36 (or 30), and the Bersa Thunder Ultra Comp 45. The Bersa is set up like a 1911, is the largest (by little bits here and there), is the CHEAPEST too. You can find them occasionally on gunbroker for under $330. But it's an off-brand and maybe hard to keep running although they get rave reviews. I just passed up a real good deal on one.
That leaves me with the Springfield or Glock. The Springfield is more costly and less rounds, but at 1" wide, that's really cool. It costs more, and it's a new design. I was burned last time I bought a brand new gun with the SR9. BUT- this is really what I want I think, so it's maybe worth the $100 or so more than the Glock. On the other hand, the Glock is just slightly larger, has one more round (6+1), and is cheaper. And I know it will work.
I think it's going to boil down to if I see an XDS in a shop and I go for it, I go for it. Otherwise, it's a slightly used G36.