For aurora you'll want something fast and (preferably) wide. Unfortunately the two don't exactly combine well if you're trying to keep your costs low. Of the two choices, you'll want width over speed. You can always shoot RAW and pray for the best in post, you can't add width for the shots themselves after the fact.
Unless you plan on intentionally going somewhere and staking out the northern lights, I wouldn't necessarily buy a lens directly FOR them. I'd get a lens you cn use for other things that will conveniently also be useful for grabbing the aurora as well - which would lean things towards the 12-24.
I'd keep the 18-135mm - while its not hugely wide, 18mm is pretty decent for catching a wide landscape and you can always use the zoom.
If I *knew* I was going to be seeing aurora and was looking for a lens out of everything out there, I'd grab one of the (comparitively) cheap Bower/Samyang/Vivitar 14mm f/2.8's. While its not super speedy, its WIDE. Yea, its all manual focus, but you don't really need autofocus for a lens being used for night time landscapes. Odds are in aurora conditions, you'll be manually focusing anyway since the AF wouldn't have anything to lock on to in the first place.
You won't really find anything else out there even close to the value for width those things offer.
As far as settings goes, it'll be a crapshoot depending on the situation at hand. The presence of a moon, the lack of a moon, light pollution, etc all will play into what you'll want to do for settings.
Glancing through flickr, it seems that a lot of the highly explored shots seem to have been shot at around ISO 800 with sub-18mm focal lengths. The exposure lengths seem to vary depending on what the photographer was after as an end result.
FWIW, I've only had the chance to shoot them once, and I didn't have time to plan ahead so was stuck with what I had at hand - my 18-55mm kit and a cheap 50mm f/1.7 prime.
The prime gave (to me) much more pleasing results simply because I could let it eat more light than the kit lens. YMMV as far as what results you may get, again, all depending upon conditions.
Here are my results from when I had the chance.
50mm, f/1.7, 15 second exposure ISO 400
(Sears 50mm f/1.7 prime)
Same settings and lens
Same settings and lens
18mm, f/3.5, 30 seconds, ISO 800
DA18-55mm kit lens