I have to say after shooting Glocks it is tough to buy any other polymer frame gun. I was at the gun shop looking lastnight looking at the Walthers after moving on from the Remington R1 and I just couldn't bring myself to doing the paperwork. Some might hate me for being biased but Glock is tough to beat and 6 yrs ago I would have been one of the guys saying 1911 or nothing. I now own two Glocks and am happy with those polys for sure!!
I would also go Glock because they have been proven tough. But out of the two mentioned here, I held the Nano and was not very impressed. I'd have to say Shield if those two were my only choices.
At the time the Shield wasn't on the MD Roster, nor have I ever handled one. If it was/I had, I may have chosen the Shield..
Yes, the Nano is odd looking and feels wierd when you first handle it. But with a full mag, it balances out well. I shot it along side a PF-9 and for me, it had a better feel, faster follow, less snap, and overall a little more accurate.
The Nano is a smaller gun than the shield in all dimensions, I believe.
I'm not trying to say the Nano is a better choice at all. But depending on your needs and tastes, it's a nice and reliable little pocket 9.