I would use the 9-18mm for the aurora, you need as wide an angle as possible. It's a brilliant lens for Iceland actually, I had this on my first trip (alongside the Panasonic 14-140mm) and I much preferred the wide angle look of the 9-18mm.
Just keep in mind it's not weather-sealed from what I remember, and Iceland in September (actually, pretty much at any time of the year) can be very, very wet and windy. I've never been as wet as on my two land-based tours of Iceland (one in late Sept/early October, one Feb/March).
On my most recent trip I had the 9mm prime from Panasonic (MicroFourThirds lenses are generally interchangeable between Olympus and Panasonic, I use Panasonic cameras but some of my favourite lenses are Olympus) along and it was perfect for the aurora, you need that very wide angle ideally.