It's a difficult argument to make, what difference is killing a whale to killing a cow, fundamentally not a lot.
However whales are not domesticated cattle. We artificially keep the number of cattle high for food, their natural numbers would be much lower. We effectively made the modern cow, sheep and other animals today, their whole evolution has been guided by us for thousands of years.
Whales are also not mindless fish. When you look into an eye of a whale he just stares back wondering what are you - a fish doesn't think anything, it just does, other mammals like cows do have some intelligence, but only other primates can compete with whales or their relatives the dolphins. And it's generally not acceptable to go around eating other primates in most places of the world today.
It's natural for us to ensure our own survival to kill other life in order to exist, but I don't believe it is right for us to plunder and destroy another species, especially one so equal to us as the whale.
The world produces a surplus of food required to feed everyone, it's true that its not very effective and is highly unbalanced. But there is no food shortage globally and there's no reason to hunt whales. Well unless of course you cave in to a few Japanese restaurants who want to cash in. Speaking of Japan, 80% of the population (from a BBC poll) are opposed to whaling in Japan.
Whales are amazing creatures, their social and communicative abilities give much insight into ourselves and we should not allow the commercial exploitation of whales.