I've been pondering this lately. I like the Kahr PM9 I got last summer, but I only carry it (in a pocket holster) on my own property. I have an older GP100, so I already know that I love the .357 Magnum round. Conversely, I'm still not crazy about the 9mm.
So to summarize....
I love the .357 Magnum round
I'm lukewarm on the 9mm
Concealment isn't a huge deal as long as the SP101 is reasonably hideable.
The Kahr holds 6+1 with the standard mag, while the SP101 only holds five.
In my opinion, revolvers are almost failsafe, Rugers in particular.