Black Friday is pretty much a store deal. So like if you buy an xbox one on black friday, in the store, it would be cheaper. Cyber Monday is online deals, so if your xbox one in the store did not decrease in price. There might be a chance that it would be on sale online.
To sum this up.
Black Friday: Store Deals
Cyber Monday: Online Deals
It all comes down to what you want and what present to buy for someone special.