decide against (someone)

To choose not to accept someone. Yes, Holly was a strong candidate, but we ultimately decided against her for the job. You lost the election. That means, the state decided against you being their governor. You decided against the CEO's daughter for the internship? Well, that decision will come back to haunt you, I'm sure.

decide against (something)

To choose not to do something. I was so tired that I decided against going out with my friends tonight. I decided against playing tonight because my ankle is still bothering me, and I don't want to make it worse. The weather was so terrible that we decided against going out to dinner as we had planned.

decide on (someone or something)

To choose someone or something, usually after a period of deliberation. Yes, Holly was a strong candidate, but we ultimately decided on someone else for the job opening. After agonizing over the menu, I finally decided on mint chocolate chip ice cream. Sure, the beach is appealing, but we ultimately decided on the mountains for our vacation this year.