Maybe it is indeed semantics, but I would tend to think that all "realists" start out as optimists. If you were not an optimist, you would NEVER start a company. I have often heard that "if I'd know what I was getting myself into.." etc. So that would pretty much rule out the realists.
Anyway I refuse to give up my optimism!
I just remember something: luck. What is luck? I believe luck is seeing a chance AND taking it. That IMHO takes some optimism. The believe that "you can do it" or that something good MIGHT come of it.