The Apple Watch Series 5 brought the always on screen. The Apple Watch Series 6 is basically the same as the 5, but with a couple more features, including faster charging time (around 40% faster than the 5). The battery life is the same at 18 hours, the design is pretty much identical, and both sport a 368 х 448 resolution display, with the same brightness.
Where they differ, is the health features of the Apple Watch Series 6. You get a blood oxygen sensor with the 6, which you don't get with the 5. The Apple Watch Series 6 also includes 5GHz Wi-Fi support, meaning that it'll be faster over wi-fi than the 5.