I personally prefer the standard "package with arrow pointing down icon." I think it is much more relevant to the action that the user will be doing. The software center icon conveys the idea of a place where many different things can be found, while this icon shows that you will be downloading a package

![16][1]

![24][2]

These are from Humanity (/usr/share/icons/Humanity/apps/16/system-software-installer.svg). Although, I actually like the ones with the orange arrow [in this answer][3] better. I'm just not sure where they come from.


  [1]: https://i.sstatic.net/rqMMK.png
  [2]: https://i.sstatic.net/trHkK.png
  [3]: http://meta.ubuntu.stackexchange.com/questions/388/the-package-installation-problem/395#395