747 - A great aircraft but now getting quite old with more and more problems appearing as time goes by. CX will start phasing out the 747 over the next few years, so the destinations will become more limited and when it comes time for upgrade you'll probably end up on the 777 anyway. Positives - can't really think of any,
777- an even better aircraft, a real pilot's machine with newer technology. The 777 will become the backbone of CX's long haul fleet as the 747 is phased out of service. Ultimately there will be a lot more variety in terms of destinations (if there isn't already). Negatives - can't think of any!