For skiing & snowboarding
For skiing & snowboarding, the two are close. Copenhagen plank rates slightly higher: more injury protection, easier to learn and fit in.
Cossack squat carries over more directly to skiing & snowboarding.
Both are good choices that build different things for skiing & snowboarding, so a plan can use both.
The research score comes from these, scored by hand from the studies. Lifter scores appear once an exercise has 10 ratings. How scores work
Copenhagen Plank. Adductor strength for keeping the legs together and in control on edges.
For combat sports, Copenhagen plank rates well ahead on our research criteria: more injury protection, easier to learn and fit in.
For skiing & snowboarding, the two are close. Copenhagen plank rates slightly higher: more injury protection, easier to learn and fit in. Cossack squat carries over more directly to skiing & snowboarding. Both are good choices that build different things for skiing & snowboarding, so a plan can use both.
About the same: both take some practice.
Copenhagen plank needs a bench, so a home gym covers it. Cossack squat needs no equipment, so you can do it anywhere.
Copenhagen plank: it rates Strong for injury protection in our research rubric, against Good for Cossack squat.