Hi Mo,
'Better' is a subjective term as both options do their job with respective pros and cons and it's usually down to an individual's preference.

So to answer your question mate, these are the factors I know of - I'm sure the guys know many more...

Composite gaskets are softer than copper, usually cheaper and can compensate for any small gouges or imperfections in the head and they seem to do a better job of sealing heads that are slightly warped.

On the down side, they do need re-tourqing several times before they seal properly and they only come in one thickness.

Copper gaskets are less yielding but on the plus side, you can get extra thick ones, which is valuable if you've had a head brutally skimmed.

They seem to benefit from being coated before fitting (I use a good thick layer of silver wheel paint on both sides) and out of the 30-odd rebuilds I've done, I've never had a copper gasket blow.

So I prefer copper.

Cheers!
Rudi