Well there are a few technical differences between the two. Mir from the start was built with "convergence" in mind, Wayland wasn't (although it can support it), Wayland after all is the protocol, not the the implementation (that is left to Gnome or KDE, user choice..). The problem is that both of them technical parts aside, do the same thing in different ways as a default. They both use the OpenGL/ES, DRI and KMS (Which Wayland is being credited for even though most was long before Wayland, Some is even the work of Xfree86, some argue as Wayland is by the same people it should have the same credits, but that's a whole different argument).
A lot of this brings back memories of the current init situation in a lot of ways, but the rolls are reversed. Canonical/Google made a great replacement for sysvinit, it was worked on continuously (even now it is still going, but people will tell you its dead or failed as it is only used in Ubuntu, Chrome and Chrome OS) it is called Upstart and started in 2006. It ties to a lot of other components like Plymouth and X (soon to be mir) but its aim is more "desktop" orientated than server (pretty boot screens, non-switching) This continued until 2010 when two Redhat devs announced systemd, which was immediately accepted as the "de facto" standard for the sysvinit replacement even though the remedy only supports Linux whereas Upstart supports Linux and BSD, dropping Upstart and Canonical/Googles work. They also included another problem systemd is packaged with files to increase its performance as standard (some rsyslog stuff), but Upstart not being an "upstream" blessed project any more does not have the files it needs packaged as standard in anything but Ubuntu and Googles Chrome OS. So in conclusion systemd on the face of it seemed faster and cleaner, although it is not "event driven". There was an amazing explanation about this at this link eli5_the_systemd_vs_initupstart_controversy
I know this is not current, but I do feel it is actually quite pertinent to the discussion, this shows that forcing people into only one supported project is not a new thing with linux, it also shows that Redhat and others are willing to turn what was accepted as the "community" solution for more than 4 years on its head and go off to duplicate the effort when it benefits them and their hold on Linux.
Both parties are not justified in this argument, but it has became another battle for Ubuntu against the two "big dogs" as it were in Linux, Intel/Redhat. The more they control the less likely it will become for linux to ever take a hold on the "desktop" market, hell they been at it for as long as X has been, they have had over 26 years to give the desktop an open source OS, up to now they have either failed or are not interested in the desktop linux market. Intel is definitely not interested in the desktop market they have Microsoft Windows (You know the thing that was such a big failure, but still holds the majority of desktops).
Now look at what happened after Ubuntu came about, a project that actually cares about Linux on a desktop as well as the other platforms and areas. It had and still has massive support from users (yes us guys here discussing this from Ubuntu machines) and companies inside and outside of the Linux/Open Source market. Canonical its investor which includes the "NIH SABDFL" everyone seems to love to hate Mr Shuttleworth, who has a direction he wants to take with Ubuntu.
Whether we all agree or disagree with the crap going around at the moment, Ubuntu even with all the changes, the mud flinging exercises and the FUD (The comments on the first announce of Mir) from both Canonical and outside projects. I do still feel that at the base of it is a leadership and community that still wants the best "Linux based Desktop distribution" around. That it wants to improve things for everyone not just Ubuntu, but it is a rather large goal..
Since 2004 when Ubuntu was released in a lot of the "end-users" cases Ubuntu has became synonymous with Linux (this is not the right thing, but neither is games or hardware manufactures calling a Windows PC just PC.. especially considering the actual PC runs a lot more than Windows). For tech savvy yeah we know the difference, for them well they need a little education on the subject.
Some of the younger users probably had the first taste of linux on Ubuntu. Then maybe they went on to bigger and better things, maybe they went to where the grass was greener, maybe they stuck around with Ubuntu and now are classed as OAPs (Like me >.> or at least that's how it feels lol). Whichever choice they made it was theirs and I hope them all the best.
And I'm doing it again.. on a reminiscing trip to who knows where. Sorry.