1. It has strong focus on real multitasking (performance and design wise). Both Android and iOS are weak on that and prefer to avoid multitasking in order to gain battery life advantage.
2. It has innovative approach to UI, not being stuck in ideas developed a while ago, which hold Android and iOS back from advancing.
3. It's DRM free and aims to be privacy respecting unlike competition. It gives control over the system to the user and doesn't attempt to lock things up and make it hard to modify.
4. For those who care (developers probably more than others) - it's real glibc based Linux which gives synergy with the global Linux community. Android while using the Linux kernel is a completley different beast from libc and middleware up to the top of the stack. Mer/Sailfish on the other hand uses glibc, Wayland and other conventional Linux middleware and tools.
5. They plan to allow better participation of community in the development. Right now things aren't so open though, except the open parts Sailfish is based on (Mer Core and Nemo Mobile which are openly developed open source projects).
6. It's a startup and not some monstrous stonewalling corp with no face. So you often can communicate with Jolla directly (they even have official presence in Diaspora: https://joindiaspora.com/u/jolla) and they are interested in the community.
For more details see:
* https://sailfishos.org/about.html
* https://sailfishos.org/design.html
It's sort of a wash from the perspective of open source. The SailfishOS core seems more open and "project-like" than AOSP, but just like with Google/GMS the core apps and user-facing bits are proprietary to Jolla. And being new, the ecosystem doesn't yet have good open source replacements you can find for Android.
I'm not sure what it gives me that Android doesn't - can anyone enlighten me?