From a technical standpoint, both Ubuntu and Linux Mint are considered "community" based Linux distributions. Both may have corporate sponsors funding the development of the two projects, but only Ubuntu takes its direction from its corporate sponsor. Linux Mint remains very clear in its independence from its sponsors.
Both distributions seek out ideas and feedback from their community of loyal users; however, neither of them has truly held itself to a true democracy. In the end, the teams who work on each distribution will go in the direction set forth by the powers that be. The community is merely a source of input...not the de facto source of either distribution's direction.
Where Ubuntu and Mint differ in direction is with long-term focus. Ubuntu seeks to become a unified operating system running on practically everything from phones to computers. Linux Mint is simply an operating system for computers—there are no plans of supporting other devices.
As I combed through this interview with the Linux Mint founder, it quickly became clear to me that LMDE is strictly Linux Mint's research and development project. While the Ubuntu base is and likely will continue to be the base from which Mint is built from, the Linux Mint developers are ready to move on to something else if Ubuntu's direction becomes too disruptive to Mint's development.
Some have argued that, in many ways, Linux Mint pays more respect to Debian than Ubuntu does. I can't say I completely agree with this statement; however, I would suggest that Linux Mint is more likely to thrive without corporate sponsorship than Ubuntu would. This is because Mint is in a position where they can roll out one sponsor and insert a new one fairly easily. Since their sponsors have no say in the project's direction, this makes Mint a more resilient project overall.
Ubuntu, on the other hand, may not be able to weather losing Canonical as its primary sponsor. Since Canonical dictates the direction of the project and pays for the developers to work on the distribution, Ubuntu would suffer tremendously if Canonical suddenly dropped out of its support role. However, most Ubuntu users don't seem to realize or agree with this line of thinking. My research indicates that Ubuntu would likely evolve into a less popular or separate project at best, if the unthinkable happened and Canonical moved on.
Both Ubuntu and Linux Mint are newbie-friendly distributions that push the envelope by constantly improving usability. However, lately it feels like Ubuntu is taking a path that makes the desktop user feel like a second class citizen. In Ubuntu's defense, they did make awesome strides in Linux gaming with their partnership with Steam. I think this is great and should be acknowledged. It's Ubuntu's push into the mobile space that has me concerned overall.
At one time, I had high hopes for Mir. On the surface, it sounded great. Flash forward to now, it seems that the Linux community couldn't be less interested in Mir and using it on other Linux distributions. Wayland, seems to be the focus for most other distributions. As for how this will affect future Ubuntu vs. Linux Mint debates, only time will tell.
But one thing is for sure, Linux Mint, while far from perfect, is likely to see more new converts in the future if Ubuntu isn't careful.