Hi Phil! In most cases, "optimist" translates to "αισιόδοξος". "οπτιμιστής" is someone who follows the philosophical movement of Optimism, which argues that the present moment is in an optimum state. Typically, an "αισιόδοξος" is someone who usually expects the best possible outcome in any given situation, one that more often than not sees the glass half full. One can be "οπτιμιστής" but not "αισιόδοξος" and vice versa. The contrast between "πεσιμιστής" and "απαισιόδοξος" is very similar. Well, I hope this was helpful!
A borrowed word is a dead word if you do not know the language from which it is borrowed. Now when I start learning Greek a lot of words in my language has started to live. However Greek is a creative language. Contrary to English and my language Swedish one can easily create new Greek words when needed. Neither the ancient Greeks nor the Byzantines were familiar with the concepts Optimism and Pessimism. In 1861 αισιόδοξος was created for the Fr. optimiste and in 1886 its opposite απαισιόδοξος was introduced and immediately accepted because every Greek could hear what they meant.
It is more impressive, intellectual and formal to use these words in everyday conversation, instead αισιόδοξος and απαισιοδοξος. Optimist and pessimist give a philosophical colour to the speech. They are not actually the same. Their use depends on the context, how formal and educated one wants to show he is etc.