If you are interested in this debate I highly recommend reading
Future Noir: The making of Blade Runner. It's a wonderfully detailed book about everything
Blade Runner.
As far as Deckard being a Replicant, I always thought we was. There are many clues in the film:
- His collection of photographs
-At one point in the film his eyes glow
-After his final fight with Roy, there was a planned scene of Deckard in an Ambulance having his fingers set and his wounds attended to. On one of the displays behind him flashes "Replicant"
-How does Gaff know what Deckard is dreaming about?
There are others, but they are already mentioned in this node anyway.