Both are equally correct, and very few arguments can be used to choose one or the other, although it is true that the Royal Spanish Academy prefers "quizá" a "quizas".
The rules set standards, but the own use and tastes of the speakers end up imposing their options. In general we can say that, moved by phonetic reasons, we tend to say "quizá" when the word that is written next begins with consonant, and "quizás" if the word that follows begins with a vowel. So, we say: "Quizá llame a Laura esta tarde", and "Quizás encuentres lo que buscas en otro lugar".
Hope I've helped you Ziya.
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