Roy, I suggest magnetic north, however be aware that if you have a compass as well there will be differences caused by all the other junk we all seem to need to mount on a dash but by setting it magnetic you ought to be able to see any major changes to either very quickly.
It doesn't matter that much either way. It won't make any real difference when you're using it other than the display being rotated by a few degrees.
If you select true north then the screen will be the same as you'd see on a paper chart. If you select magnetic then there's a chance that it will match what your compass says, but it's a slim chance!
but by setting it magnetic you ought to be able to see any major changes to either very quickly. 
