Mass effect series > DA series.
And really, Mark Darrah needs a backslap to the face because really Mass Effect handles things a lot more maturely and it has far more mature themes than Dragon Age. And Mass effect 3 is Bioware's darkest game.
While I'm curious to know what those things are, where I take issue with this part is the idea of a "mature theme"? I mean, what makes a theme mature or otherwise, and in what way does Mass Effect have more? Being a science fiction story, some themes may be incongruous to a medieval-style fantasy anyway.
As for maturity, you can tell by the romances....Mass Effect handles their romances far more deeply and maturely than Dragon Age (ex. Thane). Mass effect also handles themes on the human condition (like PTSD in all three games) far better than Dragon Age.
I'm of the opinion that Dragon Age beats Mass Effect with a barbed wire-wrapped baseball bat when it comes to companion interaction, for the simple reason that companions in Dragon Age have reactivity. That they will respond a certain way based on the protagonist's actions and choices of dialogue gives each companion greater definition and gives you a sense of their own morality. The only thing that may make Mass Effect's romances feel deeper is the fact that these romances can span for the entire trilogy, rather than existing solely in a single game. In any case, Shepard can be the biggest jerk in the universe, and no one cares.
And then there's the initiation. ME2 is pretty much the worst of the lot for me, because the way Shepard approaches these characters is really awkward. The only exception in the entire thing, in my opinion, is Garrus. For everyone else, Shepard can seem like a creepster or something. Just listen to the dialogue between FemShep and Jacob, or M!Shep and Miranda and Jack, the latter of which strikes me as a little unsettling.
The only thing darker in Dragon Age is the violence and the gore, but less mature. Theme wise, the Mass Effect series is much more mature.
Again, this mature theme idea is rather vague. Both games deal with issues of racism and slavery, but Dragon Age tends to approach it more directly with the plight of the elves. But one major issue that Mass Effect never really got around to correcting is the terribly one-sided approach to depicting its leadership. Look at the Council, and look at Udina. The central government in Mass Effect is largely ineffectual, despite being the representatives of the most advanced races in the galaxy, yet they provide little more than an annoying obstacle for the great and powerful warriors. Everyone in an executive position in a corporation was corrupt and evil.
Now, I love Mass Effect, and I've played through the trilogy plenty of times, but Dragon Age has pretty much spoiled me when it comes to its roleplaying elements.