Mirror image is useless against someone who knows what their doing no matter the race that they are. I can normally find out which of them is the real BM and which is the image of him within 1 attack of either one, course if you get them to attack me simultaniously it doesnt do well for me...

But for WW, its useless against NE if they use enough common sence to get a shop, and almost useless against UD if they do the same, since both have shops with items letting you see invisable units that its used on (get near BM and use 1 of the 2 charges, then when he tries to WW hell just get the effect but still be seen)

So i guess just pick whichever works best for you, i find most people dont know what they're doing nowadays when it comes to melee, so i guess it really doesnt matter now does it?