Often claimed, rarely demonstrated. A 7 freewheel means a non-serviceable ratchet is usually getting replaced every time the sprockets do, whereas you have to service the freehub and replace it separately every N times on a cassette wheel.
For most users, unless you do something silly like put the freewheel on ungreased threads and let it rust into place, there's not much to choose between them in practice, but cassettes are more modern and less wasteful so I think an 8 cassette is worth choosing over a 7 freewheel if all else is equal. It rarely is. I'd buy the one that felt best to ride.