Poster orientation: Landscape or portrait (horizontal or vertical)?
While I am not aware of field-specific differences, my personal experience leads me to believe that landscape is likely to be better for conveying information than portrait.  The reasons for this are:


People are much more visually sensitive to vertical than horizontal changes in position, due to the fact that we are largely surface-dwellers.  You'll have a much bigger visual impact from things that are near eye-level than things that are significantly displaced up or down.
Posters are typically consumed in parallel, with different people arriving at different times and looking at different parts.  A landscape format lets more people be close to the poster (and potentially the parts they're focusing on) than portrait format.


That said, 1) many venues restrict you tightly in the amount of space that you are allotted, and it will generally be better to use the space you've got rather than make a smaller poster with a preferred orientation, and 2) good vs. bad graphical design will vastly outweigh portrait vs. landscape in terms of impact.