I have actually owned both (I think an older C320?). Based on what I heard, not what I read in a magazine, I would take the Cambridge A3i.
The A3i was designed by Michael Creek (Creek Audio).
It also has a very good phono stage built in, so you can add a turntable. The only real difference between it and a Creek amplifier, is that it is made in Asia instead of the UK.
The NAD is a very nice amp as well, but it seemed slightly bland to me anyway. The bass seemed a little slower, which to some may make it sound warmer. But that kind of warmer came accross as bland for my taste anyway.
I would still own my A3i, except after I put it in my work office, I discovered I really needed a remote to control the volume and mute without leaving my desk. So I traded it for a Creek 4330R amplifier, which you may also want to consider if you can spare another $100+ on the used market.
They do sound about the same, the Cambridge actually has a little more power.
You really can't go wrong with an NAD either, so you will probably be happy with either.