decide against (someone or something)
1. To choose not to accept someone. Yes, Holly was a strong candidate, but we ultimately decided against her for the job.
2. To choose not to do something. I was so tired that I decided against going out with my friends tonight.
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.