We chose debian for its reliability. They also have a logical social contract:
The social contract is a promise from Debian to it’s users. Feel free to read through it.
The apt tool was originally Debian exclusive. However, many different distros now use this tool. Debian’s repositories are still by far the best out there. They contain the most packages, and it works 99% of the time. (Regardless of stable/testing/unstable)
UPDATE November 24, 2007: Personally, I stick with testing or stable for the most part.