In place of, or as an alternative to:
— Felix became a herpetologist instead.

synonyms: alternatively, or else.

Roget 147: instead; in place of, in lieu of, in the stead of, in the room of; faute de mieux [Fr.].   

On the contrary:
— He didn't call; rather (or instead), he wrote her a letter.
— Used English terms instead of Latin ones.

synonym: rather.