Relief lines, in general, are more desirable because it is a line you just don't know what your schedule is until a few days before the upcoming month - so there are no guarantees. For example, if you want Christmas off and you know can hold a reserve line with it off, do you bid a relief line in hopes of getting it off or do you sit reserve for a month to get the holiday off? Also, relief lines are assigned to the more senior reserve pilots so newhires don't get them.

It can be fairly difficult to change your schedule significantly with a weeks notice. If it's a day one way or the other, it's easier but shifting a 4 days, it's hard because you run into 30 hrs. in 7 days (30/7) issues. Most of XJT (CAL flying anyways) scheduling is done online so you can view open trips or trips advertised by other pilots online.