I recently bought a leather Zomoy 13.3" workbag for my Macbook. If you're in Europe, it could be an option. It looks great and it isn't too pricy for a leather bag.

I swear by my BBP bag for my 15" MBP, and I know they make a 13" version as well. I'm a graduate student with a traveling job for the university, and that bag holds all the books, binders, cameras, tech goodies, etc. I've ever put in it (I once used it as a primary schlep bag for a student film production--it holds about 3 formal gowns, a handful of props, and a couple handheld cameras+tripod). It may look strange, but it's mighty functional.

How tightly does your MacBook fit in the sling? I use a sleeve on my MB before I put it into a case. I may be the only one who does since few say anything about a sleeve.

The STM Rogue looks to be the most functional and probably the most comfortable to carry out of the 3. However, I would not pay $90 for essentially a backpack. Maybe $40.