Diamond Dog Food Vs. Purina?

My puppy I had just got not to long ago has been eating Diamond dog food. I know Diamond isn't the greatest dog food, and neither is Purina! But my father said he can't afford to pay for a high quality dog food! And he said something about switching to Purina dog food and I told him I believe Diamond is better! But he keeps arguing with me and he said Purina has been around longer so he must think the longer they been around is the better they are! Well im the one who has done all the research on dog food and he hasn't! So I would to hear your opinion! Would you pick Diamond or Purina? Which do you believe is the best?