I think you'll find that the Ruger SR9c isn't THAT much smaller than the XD Service model. And since you already have an XD, I voted Glock 26.
As far as other potential options, I have experience with the Taurus PT-111 (Millennium Pro) and it has been positive. I find them comfortable, slightly more concealable than the XD or the Glock, with still maintaining 12+1 capacity. Also, you're looking at saving $150. Intuition tells me this makes it a "lesser" gun, except that my experience with the PT-111 has been very positive while my experience with the G26 has been more unfortunate.
Also, if you don't mind hammer-fired, the Beretta PX4 Storm, Sub-Compact, is a SWEET pistol! I'd say it's between the Taurus and the Glock/XD in size.