Both of these words are adjectives, and although they have related meanings, those meanings differ significantly. Something decided is unmistakable, definite, or clear. Something decisive settles an issue quickly or allows one to make a decision quickly. Decided carries with it the connotation of being nonnegotiable. Decisive simply conveys increased speed in reaching the end decision.
Consider these distinctions in light of the example sentence above. Nothing about having an advantage involves making a decision, and so decisive is not really appropriate as used here. The writer would communicate more effectively by replacing decisive with decided.