I was choosing between the IS + NX, but I agree with your findings. But I'd probably may had gone for the NX, if I needed the extra cargo space. I did manage to get 45mpg from a premier NX on a test drive, but I didn't like the drive. I still like the looks though.
I managed better mpg on the IS test drive, and the drive was far superior in my normal driving habits. I'm getting about the same as you in the current weather, although I've got 30mins of extra contraflow for the next 9mths. So I expect that to get bettter after that.
I had a NX recently as a loaner, as tbh did fancy using it much, and it looked ghastly in Mesa red (just my opinion/taste though as I don't like red cars, even a Ferrari).
David.