The difference in meaning is small to negligible. There is a slight difference in nuance that is hard to precisely explain. "the pessimist" may feel slightly stronger.
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dernbuI'm quite a pessimistI take this as I'm very pessimistic.
dernbuI'm quite the pessimistI take this as closer to I'm a very good example of a pessimist.
CalifJimI take this as closer to I'm a very good example of a pessimist.Yes, I would agree.