I'd say M&P Shield. I apologize as itsn't in your choices. Why i'm suggesting it because the Shield is the rival of the Nano. I heard and read good reviews of it specially the trigger pull. I myself is looking for one. They are like ghosts, hard to find one.
You're basically asking Ford or Chevy. See if you can find a range that rents both guns and put 100 rounds through each and decide based on your personal experience. You won't go wrong with a Ruger or Beretta. Both are well respected guns. It is just a matter of which one you presonally shoot better.
I have both the Ruger LC9 and the S&W M&P Shield 40, both are great pistols for CC. I've held the Nano at the shop it also seems like a great choice. It is going to come down to which one feels best in your hand, as all 3 of these are made by quality firearms makers and are some of the best Sub-compact 9mm's you can buy.