Sixt has a better fleet for XCAR when compared to full size elite. I've seen plenty of S90, E-classes, and XTS. I believe the 5 series got phased out.
Full size elite will probably land you in a Q50 - there are plenty of those.
I'd go with Sixt - The app allows you to "pick/reserve" your specific vehicle when you are 30 mins the reservation pick up window. If you don't see what you like, then ask the counter agent or jump to Enterprise.