Landstar is a great builder! They are also closing out their single family homes division in the Dallas area, so you would get the last of a great breed. This could be a great factor for you when you re-sale the home. They want to focus more on town homes and commercial in this marketplace, so that's where they are channeling their energies now. They are a much more contemporary builder than Drees. Their product is pretty unique and difficult to find in the area. They also build in Cypress Creek in Frisco with Drees.
That's also a good factor because the homes will look different, there will be contiguous architectural integrity and you don't have to rely solely on Landstar to finish out the community. Drees has a good reputation and builds a solid product.
I've negotiated with Landstar before and helped clients save a bundle. If you haven't already got a buyer's representative representing YOUR best interests, I would so advise you to do so. It costs you nothing, but could save you a fortune in time, headaches, heartaches, not to mention your pocketbook.
Happy to help,