The last one might be better for short-term benefits. After all, you need good content in order to encourage users to exchange their credits for them. However, I don't think the credits are valuable enough if you're planning to attract users with money. Still, I guess that's fine as long as you don't focus on it. I saw a lot of projects failed because of this. When your platform gained enough traffics and there's a ton of content, your first idea would be better IMO.