Stamina is not weaker than Magika, they are for different purposes. Both of them improve the damage of appropriate skills associated with them. Magika skills tend to be slightly more potent but this is offset by the fact that stamina builds get increased white damage as well as stamina skill damage. Do not underestimate the importance of white damage as it can account for quite a lot (particularly in longer fights). Not to mention stamina builds have the option to block more and run farther, no need to explain how this can be very useful in PvP.
Both are completely viable, and that has been demonstrated by many high level players who were owning with stamina builds just as well as the magica builds. The question is how you see your character, if you are focused on hack and slash or bow damage then more focus on stamina will give you the most benefit. If you fancy yourself a power user, lots of “pew pew” and your weapon is just an auxiliary tool then magica is where you want to focus. Each one is the OP one for the right type of player.
Healing is one area that Magika has clear superiority. The vast majority of healing abilities are magika based so in this regard Stamina builds might be considered weaker as they do not have that versatility of being very effective healers. However they do gain versatility in having higher mobility and survivability. So again it comes to what each individual values most.
P.S. In the early game stamina is weaker because you have unlocked so few and mostly trash abillities for it. But this changes fairly soon, and blocking becomes much more important fairly early. Re specing is quite expensive so it is likely worth just pushing through this period rather than going magika and paying to re spec