If you have reserved midsize or higher, you can take any vehicle that is not marked as a paid upgrade; you can also expect to see better quality cars in the President's Circle area. I would also not expect to see luxury cars offered as a free upgrade option, but you may get lucky.

You can book a lower rate that still qualifies for Ultimate Choice and hope that the upgrade fee for a luxury car results in an overall cost that is cheaper than booking a luxury car outright. Or, you can take something that is in Ultimate Choice that you prefer over a luxury car (unlikely, but possible). Finally, you can just book a luxury car and not bother with Ultimate Choice.