For money Quality which is better arcam or NAD?

Arcam A18 vs c326bee. Which is more reliable?
Who knows? Six of one half a dozen of the other. "Better" is such an ambiguous term. YOU'd have to try both in YOUr system to get an answer that would satisfy YOU. Notice the emphasis on YOU?
Which is more reliable? You might want to try Consumer Reports for that answer.
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.
Bizango...Rawr is NOT free to choose between them...he's bound by principle to be guided by our opinions and, unfortunately, I don't have one.
What do you have in your system?
I have owned the NAD C340 and C350 and also the Arcam Delta 290 and preferred the Arcam. Contrary to Ryder's experience I found the Arcam to have more depth as well as a smoother, sweeter sound. The NADs were enjoyable as well (the C350 more so) but different. More punchy but somewhat grainy. Not as refined as the Arcam but more rockin'.

Hope this helps, unlike some of the other answers in this thread.
As an owner of both AND a former Authorized Service Provider of both brands I can comment on both sound and quality of build.

In my opinion, if both amps are properly biased and kept that way (the cheap-o bias pots of both brands WILL DRIFT, and must be checked every year or two), the ARCAM will not only last longer, but bring you just a smidge or two closer to the high end. I was always impressed by the build quality of ARCAM Integrated amps and CD/DVD players, just NOT impressed by either brand's HT receivers up to 5 years ago, from where I don't have further exposure.

That said, BOTH brands are committed to great sound, and since the equivalent NAD will be of cheaper cost, then perhaps one could claim the NAD to be the better value, DOLLAR WISE.
I have owned products from both brands but neither of the units of which the OP writes about. A relative still owns the NAD 3140 I bought at the PX in Guam in 1984, and I refuse to get rid of my ARCAM DV-88+ that I play exclusively CD's and HDCD's on, and LOVE the sound of. (I also was the US service center for dCS and Nagra, and never felt my measly DV-88 was terribly embarrassed by comparison to either IN MY SYSTEM).
Both are excellent, considering the costs.
I've had the Arcam Alpha 6, and still have an FMJ22. NADs: C320BEE, C325BEE, C326BEE.
The Arcam has a more extended - although a little dry - treble. It is the more transparent of the two, but also lighter in balance. Not "lean," just slightly more ethereal.

The NAD has noticeably more "heft" (more solidity in the midbass) and will deliver highly holographic, up-front, extremely well-focused images, highly musical (bold instead of pastel color palette)and rocks out. It is also excellent at microdynamics and only slightly less good on macrodynamics.
I have always steered my friends towards the NAD, and, by the way, I'm not sure I'd agree it's "grainy" at all, but it would take a speaker with an exceptionally low grain structure, such as Avalons or, astoundingly, a pair of Sound Dynamic RTS 3s, which have nearly the same continuousness of sound as Avalons and that's pretty august company to be in.
An NAD/Sound Dynamics combination is a superb combination, both of them have the same strengths.
I did have a 326BEE on a Black Diamond Racing "The Source" shelf, and I disliked the sound intensely, but later discovered it sounded grainy because I had the shelf on an Finite Elemente Spider Rack. They did not agree. That's about the only time I've heard the NAD sound grainy and the power cord was a Nordost Brahma. For $300-$500, they nearly unbeatable, in my view.
Arcam is warmer than nad to my ears, you really need to try/audition both.