I think lattitude - same constant bearing the whole time (if there was no wind etc) a line of long is a great circle route and therefore constantly changing direction, yes it goes through north, but not magnetic north!
But then again, lines of latitude are referenced to true north, not magnetic north, so the same problem applies also!
If you have a HDG(T) readout, I'd say both equally as easy - for lines of longitude hold 360/180 and latitude 090/270.
I'm sure there's somebody much smarter than I who will have a logical answer though.
