I think the most comfortable option is going to be the non-*A option - LAN, routing LAX-SCL-EZE and return. Both the outbound and return are scheduled to allow sleep between LAX and SCL. Just make sure you get the non-stop between LAX and SCL. On certain days the flight stops in LIM in the middle of the night, which really interferes with sleep. LAN is a fabulous airline; much better than UA.