Flat Earth is an opinion, and gravity is a theory. The difference is one is right and the other is wrong.
That's just an easy example of a technicality, but other opinions can be wrong too.
If someone were to say "Sonic the Hedgehog is better than Super Mario World", they would be wrong because Mario has better power ups and more levels which are better designed. Luigi's gameplay is also much deeper than Tails' abilities.