I prefer square images or just slightly longer on the verticle... I'm not sure one can say composition is more difficult within a square images area but since most experiences of a traditional nature are with the more rectangle picture spaces (35mm, 4x5, 8x10, 8.4 x 11, etc., etc.) it makes composition within a square 'different' than what one might be used to. To help with the adjustment one might seek out other visual artist's (photographers, painters, etc.) work that tend to work in square formats. One photographer that comes to mind is Michael Kenna.