I agree battery is huge for me as well when it comes to smart watches or fitness trackers. I actually got rid of one because I would take it off to charge it and forget to put it on before I left. So now I switched to one that has less functionality but uses a replaceable battery and only needs to be replaced once every 4-6 months. Since it's not considered a smart watch I won't mention it but I just wanted to comment specifically on the battery part of your post.Hi.
I'm using the Alcatel OneTouch smartwatch on my iPhone 5s.
It's compatible with Android and iOS respectively and it works like a charm.
Battery lasts somewhere in between 2-4 days on a single charge. It has basic functions and you also receive notifications on incoming calls, messages, mails etc. Google it.
But the deal breaker for me was the battery, it lasts so much more than Apple Watch.
The Pebble app has not been approved by Apple so in my opinion, Apple Watch will be the only smart watch choice for iPhones.