I see that your oldest is 12. My youngest is 12 and we are just diving into Preparing ourselves.
I've heard good things about MOH and the Christian Kids Explore. . .series, so I don't intend to knock them in any way. However, if you are looking for simplicity and efficiency in your homeschool, I do think that using Preparing for your oldest would really maximize your efforts. My own kids have always been combined in a large portion of their studies until my daughter began her 9th grade work. They have really benefited from the kind of shared study which builds family culture, and that is something Preparing can provide.
Over the past few days, I've read Preparing cover-to-cover. I'm so glad to have found it. As I said in another post, I almost ordered it for my son in the fall, but was afraid it might not be enough in some way.

I think that if I had been able to hold the manual in my hand and page through it, we'd have been using it all this year! So glad to have it now, because I know that it will be enough of a challenge for him.
Above and beyond my desire to give him an overview of world history and improve his writing skills, my major goal this year is to help him fall in love with God with his whole heart. That came so naturally to his sister, but with my quiet and reserved son I have a difficult time gauging the depth of his spirituality. In some ways I fear he is a "doubting Thomas" deep down, with a far too logical "prove it" kind of mind. Perhaps some of you know the type? There were tears in my eyes as I read the guide, seeing all the opportunities we'll have to pray together, study God's word together, and build that real love rather than just going through the motions. The amazing thing about Preparing is that I can meet his academic and spiritual needs all with one program. . .that's a first!