You are not given a choice directly. However, as both fleets recruit, just delay your arrival suiting to a course for the aircraft you prefer, they're so short of pilots so they accomodate to it. If you missed out on the 777, after 3 years you can choose the upgrade on it via the long way (transit to 777 as fo, then upgrade after 6 months). For the fast trackers (18mths) this works even without passing through the right hand seat. Busskippers can apply for a transition to the 777 aswell, after 3 years, just bear in mind that they will go without flight and productivity pay for 3 months, so they will pay for it.
For the 777 guys who really, really want a wobbly table experience, they'll have to wait for their turn onto the flying pig, meaning some 5 years for skippers, fo's wouldn't want to give up their upgrade slot for a table.