I'm not 100% sure you'd have to change the Sigma 50. That may possibly work on FF. I'm pretty sure that the Tokina is a crop lens.
Hey, if you've got the money, the 1Dx is the bomb for canon. Get it and a 50mm f/1.4 or f 1.2 and go on your trip.
Yeah I'm shooting the Northern Lights so I need the fastest wide-angle lens on a camera with phenomenal ISO performance. I went to Alaska with my wife last year to shoot the lights armed with my T2i and the Tokina, talk about bringing a knife to a gun fight. I was so bitterly disappointed with the sheer amount of noise in all my Northern Lights pictures. Soft and noisy.