I have a 2010 X5d with 3rd row seating and my father just got a 2013 X3 and I found the interior seating space of X3 to be not much different than my X5.
As each generation of X5 has grown bigger, I am hoping for a bigger 3rd-row seats (also with more space behind the 3rd row seat) in the next gen X5 as my kids have grown bigger. Would not want consider the GL350Bluetec b/c MB is just too soft.
2012 F01 750i
2013 F25 X3 3.5 - Alpine White