In terms of reliability both brands have a good reputation so I think you are free to choose between them based on other criteria that are important to you like sound, features, looks, etc. I have owned both brands and never had any problems.
I'd pick the NAD. Personally I find the Arcam to be flat sounding.
Reliability-wise, I have owned an Arcam amp for about 2+ years before selling it to another user. After about a week the new owner called me to inform the transformer blew up.
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