I would treat it as both
You gain all benefits of your elemental school, but must prepare the spell using two spell slots. There are no developer commentary on this and searching the message boards provided no insight on the topic, but I would personally interpret that the spell qualifies as both schools, your favourite school and your opposite school, something rare but that exist in the vast list of wizard spells.
There are other spells like this, like Trial of Fire and Acid, World Wave and Geyser. But note that out of these, only Geyser actually qualifies as both your elemental school and your opposed school, as you dont get to choose which elements are opposed, those are already defined. Water opposed fire and air opposes acid.
For spells that can be of multiple descriptors and the element is chosen at the time of casting, such as Elemental Aura, I would let it be prepared in a single slot, but you would only be able to select the opposition element at time of casting if it was prepared using two slots.
But this penalty can be fixed at 9th level by taking Opposition Research.