I have both Williams and Lionel aluminum passenger car sets. I think that the sets are about the same in quality, realism, etc. I really don't see much difference. Fellow train collectors can't tell the difference between the sets when they see them for the first time running on the layout. I am usually asked "Who made those trains?"
The reason I have sets from both companies is that I could get one road name from one company and another road name from the other. I also found that I could get a better deal at train shows on Williams sets that I could on Lionel. I think you pay extra for the Lionel name.
Joe