I just switched from a stainless S2 to a stainless S6, ask me anything!
Seriously, 4 years of improvements are very nice. I used to set the clock on the charger at bedtime, although it probably needed less than half the night to charge. With the S6, I put it on the charger when I wake up, and when I am ready to go to work about 1 to 1.25 hours later, it is fully charged.
The always-on screen means I can look at the time without turning on the screen. I have experimented with turning off the “wake on wrist lift” and therefore just activate the screen with a screen press or button press. I have not yet decided what I prefer.
The ECG and pulse oximeter are fine, but not absolutely necessary, at least for now. The ability to detect abnormal heart rate and fall will be of obvious benefit as I get older.
Everything is faster. It took too long for some apps to load my S2; all apps load quickly on the S6.