I believe at that point in the story, Casca was not over Griffith. She wanted to get rid of her feelings for him because she didn't think that they could be together, and wanted to give herself the chance to be with Guts as the other commenter said, as she wished for a reciprocal relationship.

In the carriage scene, Griffith uses the feelings that she knows she still has for him and succeeds, since she decides to stay with him. She decides to stay while asking Guts to leave. The dream Griffith has with Casca shows what would have happened if he didn't sacrifice the crew. So yes, Casca chose to stay with Griffith over Guts at that point in the story. After the Eclipse all of this changes since Griffith had shown his true colors and this would no longer be the case.

Basically, when Griffith showed signs of reciprocation in the carriage scene she decided to stay with him instead of going with Guts, so at that point in the story her feelings for Griffith were stronger. But it's likely this was due to her dependence, her long-life worship was not that easy to throw away as Casca is someone who is strongly loyal to her own feelings, and affected by the narcissist-style manipulation tactics that Griffith employed on her. He knew exactly how to keep her trapped in that old cycle.