If you want a fitness tracker that lasts for ages and is very easy to use, the best choice for your display is a) have no screen at all, b) use a very basic LED screen, or c) use an e-paper one with a front light like you might get on a classic Casio watch (or a Kindle, if you like).
The benefit of the various e-paper-style screen technologies is that they hardly use any power, and you can keep the screen on at all times without them lighting up dark rooms like a mini torch.
We'll see some pretty exciting developments in this area soon too. Ultra-low-power screens generally rely on reflecting ambient light rather than using a backlight or lit pixels (like OLED), meaning they use just a fraction of a single per cent of the power of an LCD display.
One of the pioneers in this area is Japan Display, whose latest wearable-ready display gets you 1.34 inches of round screen with a pretty sharp 320 x 300 pixel resolution. We'll take one of those, please – it seems to be a generation ahead of what something like the Garmin Fenix 3 uses.