I've never tried airbrushing but if I want an instant tan, that's what I'd try before ever doing a mystic tan for me. For me, mystic tan was an absolute nightmare. They tell you how to stand while you're getting sprayed and afterwards, they instruct you to wipe your body (and not dab/blot off the spray). I thought I had done an even job of wiping my body only to discover that I made massive streaks on my legs. I did the mystic tan so I could wear a skirt. I went from slightly tanned to a medium shade and I tried to fix the streaks with an instant home tanner. Nothing helped, I had to wait several days until it began wearing off (I even tried applying oil on it to remove it right away). The other gross part was that it came off unevenly on my skin in sections, I had to completely cover my arms and chest up because I had patches, I had to use bronzer on my face since it was so uneven. Horrible experience that I would never do again.
The worst part of this was that I needed this tan for a modelling assignment yet I had botched up uneven legs.
I dont think airbrushing is that much more. A gf of mine did the airbrushing paid $50 and had phenomenal even results, whereas I paid $30 for the mystic tan and had a horrific experience.
The other alternative is to use Neutrogena's Build-a-tan. I've tried so many home tanning lotions, from cheap brands to expensive brands (I must have over 10 bottles of tanning lotion pretty much full from trying different brands) and the Neutrogena one I found to be the best.
Do be careful around hands and feet if you do opt to use the home tanning lotion. If you're doing your feet, make sure to get the lotion everywhere, like around the toes, rub it into the sides of your feet. Get someone to do your back, and for your hands, what I do is wash them after I apply the lotion every where on my body then put more lotion on the back of my hands and I use the back of my hands to even out my hands. I hope that made sense, pretty much I rub my hands around to apply the tanning lotion on my hands (again, around the fingers, side of hands).
The only risk again with doing it so close to prom is that it takes practice to get the tanning lotion on nicely/evenly. I didn't get really good at it until after about 5 applications.
If you want things to go perfect, I'd say your best bet is to try airbrushing.
Sorry for the poor grammar/spelling, it's late at night.
Hope this helps.