Very limited experience, but for what it's worth: I've only seen the aurora once and used Provia 400x (with a full moon my best exposure was around 20 seconds at f4) with good results. I generally like the palette of Velvia 50, but for night work I find it too slow with too much reciprocity failure, so if I had the opportunity to experiment I'd try Velvia 100, which I like for moonlight. The aurora I saw faded pretty quickly though and I'm glad I had 400 speed to make a few different exposures before it vanished.