Are you driving between LA and LV or are these two separate rentals?

You seem concerned about the MPG - what is the estimated MPG of the other choices?

At current fuel prices I can't imagine the difference in fuel used between the sedan and the SUVs would be more than $100, probably far less. So unless you're very budget sensitive I'd just go with the type of car you're going to enjoy driving the most.

For me, that would be the largest SUV, as I've only driven SUVs since getting my first one in 2002. I enjoy the increased visibility from sitting up a little higher and don't like driving sedans where I can't see beyond the car in front of me.

If you don't like the model they assign you, you can most likely ask for a lower category so that gives you the most options. Even though the prices are similar now, you certainly can't reserve an Optima and then ask for a standard SUV when you pick it up.