I think you might be over-thinking this.

Messenger bags are great for actual "messenger-ing", but I found them to be just a big cavern for all of my stuff to get jumbled up when used for basic riding or commuting. Actually, I use my Chrome messenger bag more for a weekend bag for short trips than on my bike.

Remember, the main selling feature of the messenger bag is the ability to swing it around to the front to retrieve items quickly without the need to remove it. Personally, I don't have many (if any) situations like that. But, it's understandable that messengers do.

Basic backpacks are a really good option. They are:
- Relatively cheap compared to messenger bags
- Easy to find
- Come with a plethora of functional options
- Lots of colors
- Plus, you probably already own one!

People say that, "Backpacks suck because when leaning in the deep drops forward the bag rolls around on my back." To this, I say two things:
1) Tighten the straps of the backpack as you would a messenger bag
2) Choose a backpack with a waist strap and use it