Relative to the Series 4, there are four new things on the Apple Watch Series 5. The first is that Apple is offering new materials for the casing. You can get it in the standard aluminum and steel, but you can also spend more for titanium or ceramic now.
There are some subtle weight differences on the more expensive materials, and they also have sapphire glass on the front of the Watch. But you should not spend the extra money on those more premium materials in the hope that they’ll be better from a feature perspective.