I think playing at The Circle is the best option for all. Although Craven Park is a bit of fortress it has seen better days. I would think the opportunity for extra revenue and a full KC is too good to turn down.
Seems playing at MKM is purely down to crowd size and keeping it in 'Ull. Hull KR explain Hull FC venue swap for Brisbane World Club Challenge - BBC News
I must confess I have never been to Craven Park. Having said that I have seen numerous games on Sky and it is hardly a state of the art facility. Not a criticism but an observation.