Asking the veterans.

[Revan]

Now the topic: Keeping a balanced team or focusing power onto specific heroes... is one way better than the other, or is it simply player preference?

I personally believe that it is player preference, but some heroes are more effective for either offense or defense. Your main hero can usually go either way or somewhere in between.

Heroes at the front should be built to last awhile so that your heroes in the back get more attacks off. The heroes in the back of course will want to focus on attack.