It depends on class, build, and role.

Magicka is typically more versatile and can perform a variety of roles like healing, tanking, and damage dealing, while Stamina focuses more on damage dealing and tanking as it doesn't really have a lot of heals for healing other people. Some exceptions exist of course, but you won't find very many, if any, stamina healers.

This being said in PVP stamina currently dominates the battlefield when it comes to raw numbers. I play a mag build, and since grass always looks greener on the other side I might be biased, but some stamina builds are downright terrifying and are capable of cutting through people easily. This is further made a reality with stamina builds using a 2-handed weapon which is probably the best PVP weapon in the game thanks to its amazing Execute and universal spammable. My magicka Dragonknight has been able to compete with them though so its not cookie cutter in which is better when skill and builds are added into the discussion. Also magicka builds tend to excel in PVE, especially solo PVE, thanks to magicka's ability to easily heal itself.

Another thing to point out is that stamina builds tend to lean more on weapon skills while magicka builds lean more on class skills. Thanks to the Fighter's Guild skill line stamina builds also excel at weapon damage boosting, while magicka builds excel at max magicka boosting thanks to the Mage's guild skill line. The two play styles tend to vary wildly in their strategies as a result of this.