Apple

The differences in the SS models come down to size, not band. Bands are replaceable. Get the size that makes you happy 41 or 45mm. Me? I’d get the SS. They have sapphire glass faces which are far more resistant to scratches, and the SS body itself (plain SS, not colored like graphite) can also be polished if scratched up.

S7 vs S9? The differences are incremental. The comparison is more if the S7 is new in box to new S9. If it’s a used or refurbished S7 (unless from Apple directly, as their refurbs are like new with a fresh battery and unmarked body and face) to a new S9, consider things like battery health and presence or absence of a warranty to make a more informed decision.