Just a little update as in the meantime we picked up the car and already drove 2000 miles with it. We arrived in Seattle and they offered us a Mercedes-Benz GLC, which is what we ordered. When we asked if they could upgrade us to a GLS, they said they unfortunately didn't have a GLS at the moment, but said they could have an Escalade ready in less than 10 minutes from their other downtown branch. We declined as we'll be doing more than 10.000kms and it's not very friendly on gas. So in the end we ended up choosing the GLC, which is a very good car. All in all great service from Sixt Seattle!