I usually carry both on my Cipher/Rogue whatever if I'm making them go guns, which works out nicely as they fall under the same Focus. Blunderbuss it's generally the mainstay, as the normal DR penetration one can get is sufficient for most mobs for it still be spicy. The pistol is only brought if I really need an extra shot now at something, or if the thing just has crazy DR.
On a Rogue, anyways, Blunderbuss can get some funny high numbers. It won't be the best raw dps, of coruse, but it's fun seeing it shoot for 250-300 pretty consistently by endgame.