I'd suggest Copper will be better for you on a busy weekend.
Historically Copper has minimal lift lines on the terrain that serves the advanced and expert runs even on the busiest weekends. The lifts that serve the intermediate and beginner terrain will get crowded, but once you get away from the base you'll be fine skiing more difficult terrain. Hopefully, the capacity from the new base lifts will help the beginner and intermediates, but we'll have to see how many more skiers show up due to the IKON Pass.
The much touted "naturally divided" terrain at Copper is a huge advantage for an advanced or expert skier looking to avoid lift lines. There is always somewhere you can ski with zero lift lines. Just ski the mile-long bump runs off the A-lift and you'll never wait in line. Ski the Resolution, Mountian Chief and Blackjack lifts and you might wait a minute or two. Super Bee, Excelerator and Sierra back up to a maybe five-minute wait at the worst times, but only occasionally. Storm King frequently requires a bit of a wait on weekends, but not terrible and it's worth it. There is a lot of lift capacity for advanced and expert skiers, and most of it goes underutilized.
The same isn't true at MJ. Even the Challenger lift gets long lines on the busiest weekends. Eagle Wind is probably your best bet when crowded?
In addition to the different lift configurations, I think the Denver Hordes very much avoid driving through the tunnel on busy weekends. Day trips are much more tenable over Berthoud pass to WP/MJ than through the tunnel to Copper. No proof, but I believe this causes many front rangers to pick a WP Pass over a Copper Pass if they don't buy IKON. The tradeoff is they pay the price with lift lines when it's busy.
Because of kids schedules, I've been stuck skiing weekends recently, mostly at Copper. Lift lines have never been that bad once I'm away from the base to the advanced and expert terrain.
Here's a very long boring POV video tour of Copper in March. I'm pretty sure this was the busiest weekend I saw last season, including President's Weekend. I had to wait in a 15-minute line that morning to get away from the base but never waited in a substantial line after that. You can see there's basically nobody on the runs if you get off-piste. Then at the very end of the video, you can see how crowded it is at the beginner base when I pick up my kiddo from ski school. Thankfully they have put in two high-speed quads there in the last few years.