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
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 better. I'm just not sure where they come from.
