I've got a similar question - since we just switched from Hertz I'm not quite sure how things work with national.
I've got my first rental with National coming up but I'm not quite sure I fully understood how the selection/upgrade process works (I've got executive status).

Currently I'm seeing mid-size and full-size renting for the same price, a 3$/day surcharge for premium and 10$ a day on top of that for luxury - all sedan rates and not SUVs (both compact & standard SUV are pretty much the same rate as luxury).
Since we'll have quite a bit of luggage I'll need a full-size car or an SUV with sufficient space in back. I'll be getting the car at EWR so there will be an executive aisle.

If I understand everything correctly renting either mid-size or full-size would get me access to the ES and basically offer the same selection of cars.
How is the selection & the upgrade handled for the better categories?