TBH I'd rather they'd kept the Daleks for the Xmas show this year, since December 27th is *their* 50th anniversary...
Who says they can't do both? If they are only used in flashback in the 50th, that frees up the Christmas special to be a proper Dalek story. And hell, the marketing alone. "This Christmas on BBC One: Daleks, regeneration, new Doctor. Doctor Who: Holiday of the Daleks, Christmas Day on BBC One."