The wartime Nortons seem to be pretty reliable and robust. They are a little more old fashioned than the M20 in as much as they were based on an earlier bike (1935) The M20 was introduced in 1936 and then improved in 1938 so had some later thinking applied to the design.
In essence they are a good example of two ways to do the same job.
Being an out and out BSA man I would chose the M20 every time and I think the BSA does have some better features from the design and maintenance point of view...But the 'proof of the pudding is in the eating' and the Norton works pretty well..and any 'Norton Man' would probably take the opposite view to mine!!..Ian
email (option): ian@wright52.plus.com
I've just choked on my coffee Ian, did I read that right, that was almost a compliment to Norton 16H's 
The exposed valves on the Norton must have been a nightmare in the desert, but a 500cc sidevalve single is about as basic as you can get no matter what the make. All good fun 
email (option): horror@blueyonder.co.uk