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.
Just kidding. If your budget allows it, get the 1DX. There really is only one camera in that class imo.

