Cassette is way more durable and weather-proof than freewheel. Also cassette does not wobble like freewheel so shifting is always perfect. Cassette is also 'future-proof' in a way.
So for long term, cassette is the only choice.
The smallest size gear on a cassette is 11 tooth or even less while on freewheel is usually 13 tooth making the difference noticeable. There' also some sort of a 'cap' over the smallest gear on a cassette and finally, a rear hub with 'quick release' skewer or 'thru-axle' is more likely to be cassette. Freewheel often uses nut.