I never installed the Lollipop upgrade on my S5 because I didn't have enough free storage. Kind of glad I didn't, and with the S6 Edge around the corner, I'll just ignore the nag screens.
I'm expecting the S6 to dip a bit below the S5 in battery endurance, but not drastically so. With wireless charging, I will be able to top off throughout the day more easily. Right now, I have to open the flap to charge the phone and untether it when I get a call or step away from my desk. It's hassle enough that I often neglect to charge it throughout the day. My only concern is those times when I'm out and about all day snapping pictures and such -- that's when the spare battery really came in handy. I have a couple of portable chargers, but they're 10K mAh and kind of huge. I'll likely invest in something smaller and more portable for day trips.
The bottom line is, don't buy the S6 if your primary concern is battery life. At the same time, if the initial reviews are representative, it doesn't look like any of the Snapdragon 810 based phones (basically from LG, HTC, and eventually Sony) are going to have really good battery -- like the S6, they're going to be a step back from the previous generation. And that's without an increase in screen resolution.