I like the looks of the X. A little longer slide and grip but looks like a good gun. While the grip is linger it is not as fat. The slide has bigger groves to grip the slide with. I think the X is about $500. on GB and the Plus is about $400. Might be some deals on the Plus soon with the X coming out.
If you're going to carry the 13 round mag all the time anyway in the Plus, the only reason not to get the Shield X is that it will take some time for holster availability to catch up due to S&W finally adding an accessory rail to the Shield family.