between Mac OS Sierra and big sur which is better

Tips Big Sir Upgrade

Demand on many factors including the Computer.

Does the computer have Rotational Drive ? Big Sur will work much better with an SSD Drive

Does the computer has at least 8 GB RAM ? Big Sur is more dealing so the more RAM the better

Considering Sierra is now years out of date and unsupported, I should think the answer would be obvious.

Depends what you do & what your concerns are.