Two forms that evil assumes in the novel, why did Tolkien choose these ways to portray evil.

Sauron is more literal force of evil. That red eye sees all and manifests itself as the Biblical Satan figure. The ring is more nuanced but it does appeal to the dark side. There is a price to pay for using it. The ring eventually consumes the wearer and draws them into the dark side. The ring can be said to be an extension of Sauron's power.