No one has asked this yet, what level of a skier are you? What type of terrain do you like?
Copper is bigger and takes a little knowledge to get around, but with that said it's not like you need to make long traverses like at Vail. The trails are clearly marked and at intersections that will lead to separate base areas, there will additional signage that says which trail leads where. Copper is naturally divided, as they frequently like to brag about so you can stay in whatever pod you're in and take a different run each time.
Timberline has some nice blues, alpine has longggg bump trails, blackjack has some nice, albeit short bowls, along with Sierra. Super Bee has some nice steep groomed blacks, although they may be closed for racing.
If you're looking for technically difficult terrain, A-Basin has the east wall, although unless Colorado gets pummeled with snow this year I doubt it will be open when you come. A Basin pretty much blows all their snow early season on four trails, while blowing nothing else on any of their other terrain. A-Basin gets a lot more snow than Copper, so if it's a good snow year I'd recommend A-Basin.
Bottom line. All the long term forecasts show Colorado to have an average snow year. With that said, I'd play it safe and go with Copper.