It's affiliated with YC (created by alumni), promises to stick around and even keep your content online. Whereas Medium is still looking for a sustainable business model.
...Garry started Posthaven. Posthaven got press as the "never gonna sellout and/or shutdown" blogging platform. I still remember Posthaven whenever I think about starting a blog that I'd like to be around forever. It's a good pitch for people who value the longevity of things they write. I'm not confident Medium will be around forever, and major content sites do go bye bye: Posterous, Geocities, Vine.