meh, copper is ok but it is definitely a unique sound. I have a few and have tried a bunch of them and they have a time and place but it is hard to beat a slate call on a nice crisp morning.
the bottom line is slate is without question the easiest to learn on due to its natural sound and ease of gripping a striker. copper, glass and Crystal can all screech and slip to someone just getting into trying one.
ALL of them have a time and place, just find the one you have confidence in, using one in your living room is A LOT different then being able to run one at an awkward angle/in the woods/off your knees/with gloves/ cold finger/ and heart racing. it amazes me how different a pot call can sound in the wild rather then in your car. Find 1 or 2 you like the sound of and you are confident in and practice practice practice using a wide variety of set ups so you know you can run it no matter the situation.