@smashman42@_08's app currently has more features (horizontal scrolling mode, manga support, etc.); and at least until I do the next release fewer bugs. I'd say has a more "androidy" look and feel, mostly due to differences in the used framework.
I guess they both were born out of dissatisfaction of their respective authors with the official app, and gain features when someone requests an addition, or whenever the author feels bored ¯\(ツ)/¯.
@Icerius I'll try to add that soon along with some other sorting options. Probably not for manga, though, because it's been like half a year and I still don't know how I want to implement chapter's length/percentage storage (it stores read pages count instead of % because that's how JNC's reader does it for some reason).