Trains to Cordoba

We ( 4 people) are planning our trip to Sevilla in April. We want to go to Cordoba on the 9th of April. My question is the trains. Cost ranges from 13.25 for the MD train to 30.10 for the AVE.

1. What is the best to take? I don't mind an extra 45 minutes on the train to save over 50%

I did not see any discount for early booking for the AVE on the renfe site- Am I looking in the right place for discount tickets?

2. If we take the MD train do we need reservations?

Thanks for the help. Maureen