What is a 9s? If your referring to the Shield that's just what it a Shield. Also, the Shield is a single stack and the compacts are double stacks, and the compact does take a 12 round mag, but will also accept fullsize 17 round mags. I'd go with a compact over a Shield as you have more options.
Hi RJ, I just read a review on single vs double and the person was saying the single stack is more reliable.
Do you agree with this statement copied and pasted from Marcus L. October 19, 2008, 08:03 PM
Single stacks consist of a single column of ammunition stacked one on top of the other.
Double stacks are two columns of ammunition stacked together in a staggered manner(looks like a zipper). As the rounds are feed into the pistol they are pushed into the top magazine taper which acts as a funnel to feed into the pistol.
Single stacks are inheritantly more reliable in that they require less pressure to feed, and they put limited friction on the rounds being fed into the pistol. They are also less prone to suffering from drop jumbles where if you drop a loaded magazine on the ground the ammunition stays aligned properlyl.
Double stacked magazines give you about double the magazine capacity of single stacks, but they are "slightly" less reliable for two reasons. They are more prone to drop jumbles and the ammunition loses alignment and won't feed without fixing the problem. The second reason is that as the rounds are being fed into the top taper of the magazine the cartridges roll against themselves and if there is dust present, it can cause slow feeding speeds and malfunctions. Then of course, there is additional pressure needed from the magazine spring to push the cartridges up through the magazine taper and weak springs will cause malfuctions.
So, both designs have their own advantages and disadvantages.