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...
“One of the illusions of life is that the present hour is not the critical, decisive one.” – Ralph Waldo Emerson