Is the slide width the same for a 6906 and a 3913? Will they fir in the same holsters? I know the grip is a double stack vs. a single, but is the concealability the same?

Holsters are interchangeable, the only difference is magazine capacity. The slide is the same. I carry both and have no problems with concealment. I DO however use very high quality holster in both IWB and OWB configurations. (Kramer MSP Paddle and a Kramer #2 IWB) which help immensely.
Hope that helps you out.
Randy

The grip is thicker and the pinky extension does poke out more on the 69xx. The holsters are interchangeable as long as they both have square trigger guards or round trigger guards.

Both are great guns.

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Srigs