Checked out Sixt at both ORD and a local Chicago branch. To echo previous statements, the cars are typically fairly bare bones (by BMW standards; not by typical automakers). Here's a few examples and I hope this helps.

New 2024+ 530i xDrive w/Premium Package (head up display, gesture control, heated seats)
New 2024+ 330i xDrive w/Convenience (keyless entry, heated steering wheel)
New 2024+ X3 xDrive30i w/Convenience (keyless entry, lumbar support)
New 2024+ 840i xDrive (comes with M Sport as they all do, but no further options)

New 2024 X5 xDrive40i w/Premium & Parking Assistance & Luxury Seating Package - this I believe is the best spec of all Sixt cars as it includes the H/K stereo, cooled seats, multi contour seats, heated armrest, head up display, blind spot / 360 xam - and I checked out 5 different X5's all equipped like this. I believe nearly all of them are. I highly reccomend the X5 too because it comes standard with adaptive dampers - no other Sixt cars (except the rare to find 7 and 8 series) have that, and it has the famed B58 (again, every other BMW would be the B48 4 cylinder).

I just finished a 800 mile road trip in the X5 and got 25mpg with cruise set to 80-85. Oh, and it goes 0-60 in the high 4's... yes, this will be my pick going forward. HOWEVER, NO BMW I could see has adaptive cruise. Which kind of ruins all of this if it's a road trip.

Hope this helps!