I also have a 2013 GLK250 that I bought new, currently has 107K and has been a great car.

IMHO the 'surprise' thing isn't a great idea, this is too important a decision and the person that is driving the car should be making the call.

As you point out the GLK frame is quite a bit small than the GL - the mission profiles are quite different.

Going forward I'd think the GLK will offer fewer longer term issues the the GL.

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98 Dodge-Cummins pickup (123k)
13 GLK250 (135k)
06 E320CDI (323K)
16 C300 (62K)
82 300GD Gelaendewagen (54K)