Click to see Spoiler:Yes, Karen Gillan and Arthur Darvill are leaving sometime Next series. That said, I think they have shown that we can have two companions - or heck a male companion with the Doctor - so that helps out a lot. Will miss Rory more than Amy, but they both have worked well with this Doctor.
As far as the Christmas special - It was a bit light, and would've liked to have seen more of the workers in the story, it was a nice mix and a few surprises. Perfect for a Christmas Day show - we don't need a "Debby Downer" (SNL reference) type story, such as Tennant's final story being on that day.
That said, I hope future stories for the Doctors don't center around already established Christmas stories. If they're not careful, they'll have to try and figure out how to do a live action version of "A Peanuts Christmas Special."