I think it really depends how much you plan on carrying in it and what you'd use it for. I plan on using mine as an everday bag that I can easily take everywhere, and I don't usually carry too much (just a wallet, keys, phone, small agenda/notebook, and a makeup case with a pen, comb and chapstick in it, and sometimes an extra sweater), so I went with the medium. That way I can take it out with me at night AND use it during the day. But for a person who prefers the look of very large bags and/or carries more stuff, or wants to put bigger things like notebooks or clothes in it, the large is the thing. I think both the medium and the large are great looking (the small is too small for the style though, IMO), they actually aren't hugely different sizes anyway.