I checked out the chevy version of the enclave (equinox maybe?) before we bought the Ody. It's been a while, so I don't remember all the details of latch attachments, seat adjustments, etc. What I do remember is:
1) Once I looked at the 2011 Sienna, I ruled out the crossover because the Sienna had so much more usable interior space.
2) Once I looked at the 2011 Odyssey, I ruled out the Sienna because the seating was so much more flexible.
So I'd say the Ody is a couple notches ahead of the crossover in terms of usable interior space.
That said, the GM crossovers can offer things like all-wheel drive, a non-minivan shape, and more flexibility in picking options. If these things are important to you, and more important than the extra space & flexibility, I can see why you might want to lean towards the crossover.