* Significative emphasizes the importance of the sign.
* Significant is more general and simply means "having meaning" or "important."
Here are some examples:
* Significative: "The artist's use of color was significative of her emotional state."
* Significant: "The signing of the treaty was a significant event in history."
You can also use the adjective signal to describe something that functions as a sign, but this is less common and more specific.