Basically updated internals. They updated the memory, processor, and wireless. It is supposed to have less vibration as well.
I just got this along with the arc and it is amazing. I love playing things and turning off and on the sub to hear and feel the difference it makes,
I can't compare as I didn't have the Gen1. Also this link (https://thedigitaltheater.com/dolby-trailers/) has demos you can use to test out audio systems with the different dolby technologies.
Not a detailed answer, but I hope that helps. If anything you can search the webs on your question and probably get more detailed analysis of the two.