What's the difference between "Chłowiek jest zwierzęciem" and "Chłowiek to zwerzę"?
Both means "the human is an animal"
"Człowiek jest zwierzęciem" is more likely do be understood as applying biological systematics (human is neither bacteria nor plant, it belongs to the systematic group of animals).
"Człowiek to zwierzę" is more likely to be understood as a derogatory opinion about mankind.
I don't think there's a difference in meaning, as far as I can tell.
But in terms of grammar, using the verb być (jest) means you have to use the instrumental case.
If you just use to, then you can have both nouns in the nominative case, which might be easier for English speakers at first.