I don't know much about apple watches. That's why I wanted to consult people.
For the technology inside the watch, it's the same, except for the jump between the s7 and the s9.
aw s7 45mm stainless and steel silicone strap vs
aw s7 41mm stainless steel and milano steel strap vs
aw s9 44mm [sic: should be 45?] aluminium.
The only TECHNOLOGY difference between the S7 and the S9 is the battery life. The temperature features are used for cycle tracking for family planning purposes. For the S9 there is a "gesture control" feature, however a very similar feature exist on the S7 as an accessibility feature.
For the stainless steel vs. aluminum, that's a style and money choice.