Which AVR to purchase

Of course, you are forgoing all the newest DSP room correction technology (knowing that the deficiencies in your room acoustics are likely pulling the sonic integrity - and potential - down, likely) with these otherwise good sounding receivers.
That said, it's hard to beat the Arcam, as a tool, by-itself. It's got good refinement of detail, good balance of warmth and clarity, and quite sounding.
However, better power, body, weight, impact, transient response, etc, can be had with a modest newer tech receiver with 1.3a HD codecs and an outboard amp - not to mention the lastest in acoustics friendly processing.
But besides all that, considering reliability issues in past with NAD, I like the Arcam here. That and I can't see any receiver blowing the world away with sonic magic, so I don't even need to consider the Cambridge.

I've tried the NAD, Arcam, and now have a Cambridge. You can't go wrong with any of them. I'm very happy with the Cambridge.

See you can pick any of them and expect about the same "realm" of performance. My experience says Arcam has a better track record of maintainability than the NAD, if that helps. Cambridge, can't tell ya.
Wait, doesn't cambridge make entry level speakers and such?