Both have benefits to be fair.
Copper benefit: if the gasket is compromised it’s less likely to blow out completely and leave you stranded.
Copper negatives: FAR more prone to oil leaks. Unlike a Triumph etc, the Norton HG is sealing several oil paths. It’s a challenge for copper.
Composite benefit: far less likely to leak if fitted scrupulously clean etc.
Composite negatives: needs very frequent re torqueing after initial assembly.
There are other benefits to copper, if building a race engine etc it’s easier to get a measured tight squish with copper and you can buy different thicknesses to adjust this very nicely.
Also, if building a big bore (920 etc) copper is your only option AFAIK.
I’ve used both, I use copper on my current 920 but I use the JS motorsport Pliobond and copper wire trick. Lots of info on this forum about how to try and keep copper leak free.
On my 850 engine I use composite and have never had any leak or failure.
At the end of the day, you are struggling here with oil leaks, and are using a gasket type that is far more prone to leaks... composite would put some more of the odds in your favour.
So... why choose copper...?
