Jen, much less headroom in the X6 cargo area for the dogs. My German Shepherds can either sit with a crooked neck pressed up against the glass or stay down - but they now realize that lying down is the best option. The X5 allows them more space and headroom - so they can stand. I rarely let the back seat down - don't want them flying passed me and hitting the windscreen should I need to stop in a hurry, also keeps the cleaning of the X6 confined to the cargo area afterwards
Cheers,