Don't have the 2nd edition western but have the 1st edition western and 1st and 2nd edition combined / entire guide.
For me the differences between the first and second editions of the large guide are inconsequential. I am already up on taxonomic changes, and don't consult guidebook art in the US much, so I don't notice much difference.
Sibley guides are pretty good but I was disappointed that the second editions didn't do more to highlight field marks in the plates, and didn't include any more text. Really could fit more info about habitat and behaviour into a book the size they are, I feel. But c'est la vie, that wasn't your question.