Either Shingletown or Mineral is nice ... Shingletown KOA, although pricey, is one of the nicest commercial campgrounds you'll ever stay in ... and Shingletown is closer to Redding and north side of the park ... IF all you want to do is explore Lassen, either place will suffice

I am not familiar with Mineral. From the map I’d guess the issue might be that either location means a long drive to get to the far side of the park, so it may depend on which side of the park you want to be on.