Even 550D/T2i has 720 60p, so I would be surprised if the 5D3 doesn’t have it. The 1Dx is aimed at professional photographers as far as I understand, with some pretty impressive specs. Because of this, I think Canon can release the 5D3 with video features that the 1Dx lack and possibly be able to sell one of both models to the same customer. It will sell well if it’s significantly better than the 5D2 at video – but the potential buyers probably can’t justify the cost of the 1Dx compared to the improvements it actually has.
I saw a video about the 1Dx (rep talking and demonstrating), and it appears to have proper timecode-functionality so it’s easier to synchronize in post, as well as the possibility to remote record up to 10(?) cameras with one click. Seems like a good thing to have if you’re into making music videos, if you’ve got several cameras, that is. Let’s hope the 5D3 will feature this as well.