You don't know it's a better battery life...
We just know the 10 is 2 hours longer than an iPhone 7. Not so much how the 8 and 8+ compare to the 10.
I'd stick with the 8+ because FaceID is going to take longer than TouchID, and that bar on the top of the 10 looks horrible. I'd wait until they can embed TouchID into the screen. The 10 feels like a beta test. Wait till the 2nd iteration if you want to enjoy the experience. Unless you really need those animojis. The 8+ has the same hardware inside it except the cameras being a tad bit better, and the screen. I guess it just comes down to that screen looking terrible. If you're going to get rid of the bezels then it should look more like the Galaxy and less like the iPhone X.
How is Face ID going to take longer than Touch ID? Touch ID requires you to look at your phone, grab it, place your finger, and wait for it to read it. If any part of your finger is dirty or covered by something, it doesn't work.
With Face ID all you do is glance at the phone. And it's 20x more secure than Touch ID fingerprints are.
I mean if you think the bar at the top of the X looks bad, what do you think about the borderline-second phone sized bezels of the 6/7/8+? That shit is objectively worse, you're just used to it by now.
Also an OLED screen virtually guarantees a better battery life over something that's almost the same internally but with a standard LED screen. The only way that's not going in the X's favor is if Apple puts a significantly bigger battery in the 8+ than the X.