Personally Skrettings - they have a texture that means they soak up oils/flavours quicker if you have additives you want to use. I also find they grind really nicely into powder for additives to groundbait (I make all my own groundbait so this is important for me)
Coppens are far more sticky than Skrettings. Jamie Hughes I think recommends a mixture of the two for use on Method feeders. 100% Coppens and they will never release from the feeder, 100% Skrettings and they will not survive hitting the surface.
For loosefeed much the same. Wetted Coppens go like concrete while Skrettings will break up into individual pellets after being squeezed into a ball.
Coppens Coarse are the harder pellet that takes a long time to break down, I tend to use them for carp. Coppens make a carp pellet, which is closer to Skrettings, I prefer Skrettings where skimmers are the target (or a mixed bag).
You have two types of Coppins pellets, a light one which breaks down in about 30 minutes, these are the once Neil is referring to, which stick to a method, and the Harder one(darker ) as Maesknoll mentioned which still stays as a pellet after 3 hours in water.
I prefer Skrettings micros for the feeder and Coppins for 4 .6 and 8 mm carp fishing