Depending on your climate, locale, and drainage issues I think I would go with the engineer. Your contractor is basically building a sidewalk to place your driveway on top of. If you have that much heavy traffic running on the drive anyway the concrete will break up just as well and there is no natural drainage with concrete. At least with stone dust water and moisture will drain/wick away and leave you with a relatively dry compacted base. I personally have a 30 year old 20' wide x 80' long drive with a bed of 1" stone 12" deep covered with 4" of grade 9 w/ fines (much coarser than stone dust) with 2" of asphalt over that and have never had any rutting problems and I have good drainage from all the stone.