I'd go with the SE or 6S at minimum at this point. 6 seems like it would be just a bit too rickety with iOS 11 (thought I haven't tried it myself to be fair).
this - the ram is the kicker. 2GB ram is the new baseline- 6s/SE will be fine for a good few years I'd think. They'll be like the ipad mini 2, just keep chugging along.
This has been my experience as well, I've been ending the day with around 70-75% battery life on my (non-LTE) 38mm Series 3. I honestly thought the battery percentage got stuck or something because it seemed too good to be true. I'm coming from a Series 0 which would usually end the day at like 20% or less and this is essentially the complete reverse.
I restored from my old watch so all the settings are the exact same as are my usage habits. My typical usage is 30 min workout each day, maps companion app when I'm driving (haptics and audio for each turn), probably 100+ notifications glances, and controlling music (with AirPods).
I'm very glad I updated, it's a huge leap not just in battery life but speed too.
Out of curiosity I didn't charge my S3 last night. I'm currently at 27 hours off the charger (yesterday morning 6am) and I'm at 71% battery. It may be a calibration thing, but I'm going to run it down to zero, see if I can squeeze 3 days out of it.
Just having a solid 2 day battery will be great - no worries if you forget to charge overnight and at a stretch could manage a weekend away without having to pull out the charger from the night stand (pain in the ass that it uses a special charger, I have lightning cables coming out of my ears)