Have tweeted about this from the Snowplow account...
(By the way, m/r is times slower than Aggregation Framework, especially for simple cases or cases that fit AF's common use patterns. See http://goshakkk.name/analytics-analyzing-tracked-events/ for more details.)
I'm saying this without having gone through the code base to see what is involved. ;)
Sleek is a library that you can add to any Ruby project and use it right off. Cube is a separate service that you need to setup and monitor.
My goal is to make Sleek more of an analytics framework, that can be tightly integrated in your Ruby application and easily extended in Ruby.
Right now Sleek is very simplistic but it already does something that Cube doesn't (relative timeframes, namespacing). I'm also planning on adding other functionality that's missing from Cube (relative timeframes with proper support of timezones, funnel analysis, ability to group_by metric values, probably a nice wrapper around MongoDB Aggregation Framework.)
(I used Cube while developing ShopStream initially. As it later turned, using it as a separate service wasn't really convenient to use because of how ShopStream works and because of Cube limits. It'd be much easier to have everything embedded in the Rails app and wrap MongoDB Aggregation Framework myself. That's how ShopStream's own analytics library was born. But alas, it was very tied to the way we do things. I wanted to make something more universal so I made Sleek.)
It reminds me of https://keen.io/ and looks like you have a lot of the stuff there.
Keep the good work!