Sorry to dredge this one up, but I was asked this recently.
Surely if your flying a line of longitude then it is a great circle but is also constant heading (i.e. north or south).
If you're flying latitudes then you are also flying a constant heading (although only the equator is line of latitude which is also a great circle track).
So can anyone answer the original question: "Is it easier to fly a line of longitude or latitude?" and give reasons?
Thanks,
DB.