I am a user of both. At this time, I'd have to say the only reason I use Atom is because its markdown support is better (nicer code highlighting right out of the box and a real nice side-by-side live preview plug-in[1]).

Away from the speed thing (which is definitely noticeable on my two year old box), and from non power user perspective, I'd say they are pretty close to equivalent.

[1] https://github.com/atom/markdown-preview