Haven't held the Nano or the P290. For me, comfort is a big deal. I could not find a firearm larger than the LCP/P238/P380 (Khar)/SW Bodyguard that would fit comfortably in my front pockets when I was searching for a mouse gun.
P238 is a great mouse gun. (+1)
LCP is also a good mouse gun.
I recently got to handle a P938. That firearm did comfortably fit in my pocket, and I could see firearms with similar dimensions like the Nano would also fit in my pocket. The price of the P938 is disappointingly high though...