quoting Nessari:

How and why is pacman better than aptitude? Genuine question.

1) Packages are much more likely to be up-to-date in pacman. Ubuntu and Debian are notorious for leaving very old versions of things in their repos; a lot of programs tell you to manually install things from their own sites instead of getting them from the aptitude repos, which totally goes against the point of having a package manager.
2) Between the official Arch repos and the AUR (pacman only deals with the official repos, but there's a pacman clone for the AUR... what did they replace yaourt with? packer?)... in the entire time I used Arch/Archbang, there was only one thing I had to install that wasn't in either the official repos or the AUR, and that was a font. Ubuntu has jack shit in its repos.
3) The syntax is slightly less annoying.

The downside of this is that Arch doesn't explicitly worry about stability like Ubuntu does, but in practice that's not too much of a problem.