I don't know about LAX, but recently for XCAR I have seen mostly Audi A6 (PHX, LAS, SFO) and the odd Cadillac XTS. 5-series and E-Class were rare. A friend had booked one and they gave him an X5 instead as they didn't have any XCAR.

Also sometimes, they have your car category and even type available, but may be hesitant to assign it to you if you are taking it one-way or for a long period / lots of miles (expected). Some of the cars may be close to the maximum permitted mileage or end of the lease period and need to be returned to the dealers. YMMV.