In your opinion, which faction is the strongest in Heroes 5? Let's discuss the advantages and disadvantages of each faction.

Berserk is countered by vampirism and antimagic.

Of course, it can be countered; I wrote that it cannot be dispelled, and that's important. A dispel can easily reduce initiative by half, but Berserk cannot.
Out of habit, we apply Berserk so that it cannot be removed, at least until the first turn of the unit. Then, you remove it, and both sides have spent a turn, but only one has benefited. Of course, sometimes it's useful to apply Antimagic or Vampirism.

There's a 50% chance of catching Berserk, compared to 5/9 (~55%) of catching either antimagic or vampirism. So, what's easier?

It's closer to 56%, but how many races do you level up Light and Dark magic simultaneously? And do you level them up simultaneously for a Knight, the only unit with Light and Dark in its guild? I don't think so, so the probability should still be estimated at 33%.