First off, even in the US with National, there are no upgrades when reserving an SUV. Typically, you reserve a midsize car and they upgrade you to a full-size car, however, at airport location with Emerald Aisle, you can select any vehicle in the aisle, or if Emerald Exec, you can choose from the Exec section which typically has better vehicles. If you rent an SUV, you get an SUV--no guaranteed upgrade. That's the way it works.

As far as Madrid Airport goes, Emerald Exec may get you an upgrade but it's not guaranteed. There is no Emerald Aisle service there that we are aware of. And again, when reserving an SUV, you are not entitled to an upgrade regardless of location.

In short, expect nothing, and be happy if you get something better than reserved if you rent with National. If there are savings with Enterprise, it's probably worth booking with them since you're not likely to get an upgrade with National anyway. You can always enter your National Emerald member number on the Enterprise reservation to earn National points if you prefer.