In terms of his own development as a player, I don't think it matters where O'Connor goes, provided it isn't the Reds.
If he goes to the Brumbies, the Force should rob whatever banks are left to secure To'omua and Leiliifano as a double act. Behind what is becoming a very strong pack, outside a very experienced scrum half and inside Cross, Cummings and Shepherd, the two would really blossom.