DAC is the mothership and grants a lot more perceived security over RUK. RUK is a relatively new operation that until very recently at least was a mess, operationally speaking, as expected for a new operation. There is also the theory that once you join RUK, you will have great difficulty getting out. At the moment you can go from DAC, to Malta Air to Buzz with relative ease, that is flexibility. In RUK the company are quite stuck and it's hard to see anyone be granted approval to move AOCs (if they say they suddenly want an EU base). There is also the issue of limited basing (but supposedly the entire UK operation will eventually become RUK)