I suspect the two layers is better than the single layer. There are some good articles on how hard the designers of the first Genesis had to work to convince Mr Stephens that an gasser could cook nearly as well as a Kettle.
Then, probably, the bean counters had there say, 'fewer bars, etc, lower production cost- who will notice?'
Rich Dahl has had both types, which one does he keep? MikeBass is spot on-Which one looks better?
Having said all of that, BOTH types are Webers. Accept no substitute.