Does the plaintiff or the defendant chose to go to mediation?

It needs to be agreed by both parties. (The first step to show they are willing to come to an agreement. If they do not agree, then an order from a judge is then set.)

Its an option offered to both

Either party can initiate it. Both must agree to it, voluntarily.

The defendant if the Plaintiff also agrees.

They both have to agree on mediation. Usually the mediation fee is split, but not always. Sometimes the defendant pays the whole fee in the hopes that the plaintiff will settle at mediation. Mediation is not binding, as opposed to arbitration. In other words, it may be a complete waste of time to go to mediation, but more often than not, the matter settles and you avoid the expense of a long drawn-out litigation and trial.

Generally both parties must agree to go to mediation. It can be suggested by either party. In some cases, the court or local rules may require a settlement conference, which is the same thing as a mediation, but it is manditory and the parties are required to participate in good faith.

Local laws may vary, consult a lawyer in your area for specific details.