There is no difference between the fleets. As we do not use a bidline system, it is impossible for anyone, no matter how senior, to bid for every weekend off. There are strict limits as to how many trips or days off you can bid for, and the maxima reduce if you have leave in that month. We also use a rotating bid group system, with a balance of seniority in each of three bid groups. That way, even the most junior pilot has a good chance of getting some preferred days off or trips one month in three.
With the nature of the trips flown, and the minimum time between flights, it is possible to get into an all-weekends or all-weekdays roster, as the trip cycle is approximately 6 or 7 days. Therefore tactical bidding can be used to break the cycle and get some weekends off.
The system works pretty well (though we've been having some trouble getting A340 rosters out on time recently - hopefully now sorted), and you don't hear too many complaints on the line.