I've used both messenger bags and backpacks as well as panniers. I've never used the Chrome, but I almost bought the Ortlieb.

I was a messenger bag user for many years until I trashed my shoulder while snow boarding. The shoulder was about 80% healed within a week or two but was still bugging me after a couple of months. So I thought I'd ditch the messenger bag for awhile and try panniers.

Before long my shoulder felt good again. I knew that panniers weren't quite for me (at least for commuting), but I was worried my shoulder trouble would just come back if I started using the mess bag again. So I started shopping around. I considered a better mess bag like the Chrome but figured that spreading the load over two shoulders by using a backpack would be less of gamble.

REI had the Ortlieb on sale but after trying it on in the store I just didn't like it. Roll-top bags like the Ortlieb tend to form these little wings that interfere with your rear vision. You'll notice it in the video you posted in the other thread. The other thing I didn't like about the Classic was that the organizer pouch wasn't included. I think it is in the Pro version of the bag (I can't remember if it's actually called "Pro" or something else). The classic is just one big bag and nothing else. Having to purchase the pouch separately struck me as kind of stingy given the money your spending on a bag like that. Maybe they don't charge extra for it anymore.

What I ended up getting was a Banjo Bros backpack. The advantages it had over the Ortlieb were:

1. a few extra pockets, pouches, etc.
2. a roll top design that minimized the wings
3. a really handy side pocket that you can reach with the bag on.

Now the Banjo Bros. bag won't match the quality of the Ortlieb and I'm not trying to sell you on it, but it helps highlight what I didn't like about the Ortlieb.

Since you're debating between a backpack and a mess bag, I'll leave you with a few thoughts about the pros and cons of both.

One big advantage of the mess bag is that you have easy access to your stuff without taking the bag off. You just slip the bag around to the front, open that massive flap and grab what you want. Backpacks need to be removed and even then getting to something at the bottom often means unpacking a bunch of other stuff.

The advantage of the backpack is better weight distribution across your shoulders, potentially more padding, and potentially better ventilation.

With a mess bag, it's more important to learn how to pack it properly so you don't end up with something digging into you.

Just one final thought about No1mad's comment regarding supporting the weight on your hips. That's very important in a backpack used for hiking. I'm not sure about cycling, especially if you ride using an aggressive body position. A substantial hip strap of the variety used in hiking backpacks might be uncomfortable on a bike.