[nelly]

Not a lot in it really regarding quality as most of them come from a handful of machine shops and then get "branded".
One thing to watch for is an integral collar around the spring. Where this is present, when the caps are fitted and done up, you'll be able to see little of the spring itself.
What this does is protect the spring pillars if the bike goes over. You will scratch the cover up, but you stand far less chance of breaking the drum.