Modern hard disks have many heads. Switching between heads of the same track is fast, since it is purely electronic. Switching between tracks is slow, since it involves moving real world matter. So if you have a disk with many heads and one with less heads and both are identical in other parameters, the disk with many heads will be faster. Splitting swap and putting it on both disks will be even faster, though.
Older disks have the same number of sectors on all tracks.
With these disks it will be fastest to put your swap in the middle of the disks, assuming that your disk head will move from a random track towards the swap area. They have more sectors on the outer tracks. With a constant number of rpms, this yields a far greater performance on the outer tracks than on the inner ones. Put your swap on the fast tracks. However, see Kristian's most recent comments on this issue.