I've sat in both and know some of the specs.
If you can get a used one with air suspension it's nice because it load levels. If you look through the threads you'll see pictures that people have posted of air suspension and non air suspension towing. Obviously, load makes a big difference.
The Q7 is a little heavier so it should be slightly more stable if towing very heavy loads. It also has the 3rd seat row.
Every dealer is different when it comes to service but from what I've heard, in general, German brand dealers have a lot more variability than Japanese dealers just because employee training is more standard in the other brands.