I just received the STM Small Alley Bag yesterday. It is perfect for my 13-inch MBP. The padded section for the laptop itself is actually very big. I am still going to put my laptop in the Incase sleeve and then drop it in the STM bag. This will keep the Macbook from moving around.
It seems like a well built bag and has plenty of room for documents, cables etc. I would definitely recommend the small bag if you will be using it for the 13-inch.
Good luck!