Williams aluminum cars are difficult to distinguish from Lionel cars. I would note that the Williams vista dome cars lack the interior detail that the Lionel cars have. If this bothers you, that is one difference. The parts for the Lionel vista dome can, however, be retrofitted to the Williams car but the price gap between the two may diminish considerably. I added an interior to my Williams Pennsy vista dome solely because I wanted to do the project – I had bought the cars years earlier and it was a fun project. The painting and decoration on the Williams cars is top notch.
If you are comparing new to new, I would prefer the Williams. My recent experience it that while Lionel is doing a great job on high end locomotives, Williams offers better quality and pricing on the postwar reproduction items. This is particularly evident on the plastic 2400 series plastic passenger cars.
I would agree with everyone here that Williams cars are the better choice and the price point is outstanding. I have a lot of their passengers cars and they are very good in my opinion.