my last vehicle before i got the y was an MB e300 sedan so my comments are a little different. the mercedes are gorgeous. but i found the controls vastly over complicated (and i'm a gadget and computer geek for many years) the sound system is much better though the AMG probably has upgraded sound.
as far as room, i think the Y has amazing leg, head, and general room for 4 people to be VERY comfy. when the seats are down there is plenty of room compared to my previous vehicles which included the acura RDX, a GLK, rendesvous, and santa fe. the only thing it suffers a bit from is the sloping hatch, which mercedes and bmw also make. but the glc43 is the more traditional shape.