Flaps Down, CX has 747s, 777s, A340s and A330s. Strictly a Wide and Heavy airline. The S/Os are put on the 747s and A340s. Of course you can tell them which aircraft you prefer but ultimately you are placed where they need you.
As for rostering and lifestyle regarding the aircraft to which you are assigned, I don't think it makes any difference as far as a S/O is concerned. The reason for this is that the A340 and 747 fly many of the same routes but at different times of the day to accomodate the different levels of demand(I read of this on Cathay's website press release section). The 747 flies when there are more pax to transport. Of course the A340 flies into places that the 747 doesn't such as Rome and Toronto where there is just enough demand to even warrant the A340. Therefore you fly to the same places(generally) but just at different times.
Once you achieve the rank of First Officer you can bid on the 777 and A330 postings.
The info I have provided was found by purusing the cathaypacific.com press release website and reading the last few months of forum discourse related to CX.
Hope this answers your questions.
CJ