If I were after a smaller 9mm i would look at the following guns. Much of it subjective. However, assuming that the particular one in question is in good operating order there is not a bad choice among them and I would carry any of them.
The kahr K/P/CW 9: This gun is just about the perfect size carry gun IMHO. It is large enough to shoot and manipulate it fairly easily, yet small and thin enough that it hides well under just a T-shirt. Price wise the CW9 is a winner. It sales for $350 new around here. It is a very good carry gun period and real hard to beat at that price.
Walther PPS: This is another gun that falls in that sweet spot size wise.
Glock 26: This is one that I very frequently carry, It is a bit thicker than the guns above but not much. One nice thing is it takes mags from any of the larger glocks. Also glocks have far and away the best aftermarket support. There is an argument that if you can carry a 26 then a 19 is just as easily carried. I think that is often true with any of the sub compact versions of guns that have slightly larger compact versions. That said, I shoot the G26 as well as a 19 and it does offer some added versatility and sometimes being only a bit smaller makes something conceal much better.
Springfield EMP: A Savy shopper could get a CW9 and a G26 for the price of this gun. It is not cheap. However, it is a nice gun and a sweet shooter. If one likes 1911s this is a very nicely sized gun and easy to carry.
Steyr S9: This is a gun that is similar to the G26. I always believe that anyone considering a glock or other striker fired polymer gun owes it to themselves to to give a hard look at the Steyrs. I personally find that my steyr compares very favorably to my glocks. I like the grip better, the trigger better and the extremely low bore axis makes for a soft shooting gun and fast followups. The first time someone recommended a steyr M9 to me, I didn't even give it any consideration because I had never heard of it, I was after a Glock, an XD, or a S&W M&P. I am very glad that a year later I actually started to investigate it and then bought one. I've had many people that claim to loath glocks shoot my steyr and like it. Some even bought their own afterwards.
The shield looks interesting, and is likely a good gun, but is new enough I would pass on it for now, particularly given the existence of the Walther and the Kahrs.
I'd look at the XD since you have a bigger one.
If you want something much smaller then I'd consider the Kahr Cm/PM 9.
