To help you out:
Ashley is more sophisticated and is more likely NOT to be seen in a pair of jeans, while Mary-Kate prefers jeans & a t-shirt and also has more of a "unique" style or sometimes a "punk" style. Ashley is also more likely to be seen with her nails done & something done with her hair while Mary-Kate is not. I've also read in interviews that Ashley is more responsible. Ashley is the one who does most of the talking in interviews as well.
As for looks, right now Mary-Kate has the darker hair & Ashley sports the bangs with the blonde. Ashley's facial features are more "fuller" than Mary-Kate's & Ashley is taller by an inch or two. If you are looking at older pictures, Mary-Kate was usually seen in the blue or purple while Ashley was in the pink or the red. Ashley's hair had a darker tint to it and she had a small freckle above her lip, but it seems as if it faded as she grew older.
Hope that helped some of you out!
...I personally prefer Mary-Kate for her sense of humor & better acting, but Ashley seems very mature for her age, so I'll give her that.