If you must have a mid-size SUV, then that's what you should reserve.

If you want to take your chances (pretty good IMO, but check the location-specific thread in the forum for recent reports), reserve a mid-size car. When you arrive, you can choose from the ES or Aisle. Or ER-Midsize for that matter.

The risk of course is that there's nothing in those areas that meets your needs/wants. In that event, you could go to the counter and request a mid-size SUV, but you may be charged a much higher rate, walk up-type for it.

Also, since you mention being new to National, I think you need to present yourself at the counter for your first rental regardless. Could some recent-enrollee comment?