Personal preference really. I always say the Kuterlite compression fittings are the best by a mile and they come with brass olives and I have never had one weep.
I think the copper ones are more prone to overtightning and moving out of shape. The brass flowflex olives on Screwfix are great reasonably priced and you don't need to be King Kong to nip them up. I do agree that some of the brass olives are carp.