It's pretty difficult to make a decision when it comes to distros of DEs...
Canonical / Ubuntu does bring much to Debian to make it easier and work better overall (... when they don't push stuff that are alpha quality to get them ready for the next LTS...). Linut Mint is "just" mainly a couple of guys (one in particular) tweaking Ubuntu to get rid of the bad decisions that are made in it (which is important as well). (oh, and there is the LMDE experiment too)
(for instance, they stuck to Gnome 2 as long as possible and now try to bring the Gnome 2 look and feel to Gnome 3 instead of either switching to Unity or Gnome Shell).
Both play a role, but what Canonical does, and what the Debian community does is extremely important as well. (that's why I think it's good to subscribe to U1 to help fund it).
If you choose Unity (based on G3), the current Ubuntu version is pretty buggy (neither Gnome 3 or Unity are that stable yet -- usable though). The next LTS will probably be very good, in april 2012.
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However, to add even more complexity, there is KDE 4. I wish Canonical decided to use it instead of Gnome and add some polish to it. For KDE, the best you can do ATM is either use OpenSUSE or Arch (but in Arch you will inevitably have regressions from time to time). And there won't be Canonical to do the "papercuts", backports, support, bring better fonts, easier configuration, commercial apps... Used it for a couple of years and I finally got fed up.
To sum up : TOO MANY CHOICES (G2, G3 + shell, G3 + unity, KDE, + all distros combos), that's killing me (as, obviously, none of them is perfect)! Kinda reminds me of the Amiga community with OS4, AROS and MOS
