Can speak from experience here, we really liked the GLK. I have a great relationship with our MB dealer and truthfully would MUCH prefer to buy from them than anyone.
The GLK is the most comfortable of all cars we drove. Very smooth. It picks up well, but to me being an NA V6, even with 300hp, the drivability just isn't there. Feels much more normal and boring. X1 35i is much faster, I would put this car about on par with the Q5 3.0T in terms of acceleration. But again, to me these aren't performance vehicles, they are crossover suv's with a bit of flavor. The updates to the 2013 GLK are very nice, the AMG exterior package sets it off terrifically, in fact looks wise it was my wife's favorite. The interior just is blah compared to the Audi, and driving experience well short of the X1.