My new 2020 MacBook Air arrived yesterday. I replaced a 2018 model, which I successfully sold very quickly on eBay. As others have said, in many cases it is the same - same screen, same ports, same size. However I changed it because of the keyboard - I found my 2018 model was adequate but my old 2011 Macbook Pro keyboard was so much better. I appreciate that the internals are also different. The new model goes back to the old “feel” on the keyboard and is less “hard on my fingers” for someone like me who very quickly touch types and probably quite firmly.