Depends on what you seek. If you want to party and don't mind crowded beaches, Goa is the place. If you're looking for more secluded beaches and a peaceful atmosphere, South Goa or Gokarna are your options.
Goa is undoubtedly better when it comes to water sports & adventurous activities...
But if you are a beach lover & want to experience some good beach scenic view,Gokarna is surely a must see
The beaches in Gokarna are a way more beautiful as compared to Goa this days..
& you can even visit Murudeshwar Temple if going to Gokarna
If you are going alone gokarna would be a better choice as you would meet peaceful and like minded people and gokarna is damn beautiful . After which if u have time can travel to goa for a day ..Goa is best with a partner or a group of friends .
Gokarana if u want peace, want to see untouched nature, clean environment and great scenic views ...
Goa if u want to see the crowd, great views, cheap booze and stays.