That is a good question. Personally, I would prefer them to be cockerels - they get bigger than the pullets do, and they don't start their fighting until 17 weeks or so, and it doesn't get violent until after 20 weeks. From my chart of the last batch, the males are generally bigger than the females from week 1, and they continue to be bigger. At week 12, my WR cockerels were 44% larger than the pullets, and Dels 34% larger than the pullets. I also kept the pullets around an extra 2-3 weeks longer than the cockerels to let them fill out a bit more before dispatch.
I could see preferring females if you aren't thinking they will be ready by 20 weeks. I'm not sure if this is true or not, but it seems to me that the males mature faster than the females, so the males would get tougher at a younger age than the pullets.
I've read a bit about forcing more of one sex to hatch based on incubator temp (+/- a degree), but I haven't tried it. It's basically killing the eggs of the sex that you don't want by manipulating the temp for the duration of the hatch. I haven't tried it yet, but I will definitely give it a go with my next hatch in April. I will probably start out with about 24 eggs, if half are female and die due to the temp, that leaves me with 12, and likely 3-4 of them won't make it (in my experience). I've also heard of forcing more males by adding Apple Cider Vinegar to the parents' water. I'm not sure if I believe that, but I may give that a shot at some point too.