some differences I've found
under directx, the left and/or top of the bounding box aren't visible when you move it all the way to the left and/or top. under opengl, they stay visible.
under directx, if you use the poweruser plugin's zoomIn and zoomOut commands that adjust the font size on the fly, memory usage shoots up 2x to 3x. no impact under opengl.
alas, gamma correction of the minimap doesn't appear to be working in opengl
FWIW, I'm using windows 7 with a very recent (august or sept) update of directx.