I was going to buy the same size as the one you are thinking about now, but the colour was what stopped me. I fell in love with the colour Cameo (nude pink/beige) and because it was a neutral colour that I could wear everyday, I felt like I needed a size that would suit me for everyday, so I went up a size. I decided that the small one looks best for bright colours (it adds a cute pop of colour for that size bag).
So, I think the size looks best with a bright colour.. Go with orange! Save black if you ever get a bigger work/everyday bag.