Rental car companies: which can I choose my own car?

I have rented a few cars while I am in LV, but only have used Alamo and Budget. I like the ability to see prices on Costco travel, so have used that exclusively. I know that Alamo lets you pick any car out of your class, and Budget pre-assigns the car to you, but what about Enterprise? The other question: Alamo has moved the Toyota Sienna into the 'standard van' class at a price of about $80-100 more than a mini van. What are my odds of asking nicely for a complementary upgrade to that van?

 

I've used Enterprise a few times in Vegas (including my next trip on Sunday). I've usually had a choice of 4-5 different models depending on which class I booked. I was fine with the options.