TRANSFERRED FROM DISQUS COMMENTS:
Another important difference between Debian and Ubuntu is that Debian tends to look to the community for overarching technological strategies whereas Ubuntu has a history of trying to impose strategies, with somewhat mixed results. Upstart, MIR, Unity (desktop manager), Bazaar, Juju and Snappy are all examples of areas where Canonical attempted to lead the community instead of allowing a leader to emerge organically. There is a degree of technical risk that comes with this approach.
Ubuntu provides user a polished Linux distro, debian on the other hand, provide user a Linux distro based on free software; Ubuntu cares about user friendly, debian care about freedom.
If you have time, debian can offer you more learning space than Ubuntu; Ubuntu is better suited for lazy people like me.
But I really hate Ubuntu switching main DE to Gnome and killing i386 supports. Maybe someday I have to allocate my time for Debian i386 installation.