UPDATE: After a few years i still love them both! I have a long-term review of both of these bags coming in a few days! stay tuned.
I could not find anywhere on the internet a review of these two bags so I decided to buy them both to figure out what it was that I wanted. Both bags are premium bags that will protect your gear and look good doing it! the value of the bags to you will depend on what you carry other than your camera. The ONA is larger and accommodates a laptop with ease. it also can accommodate a small DSLR and 2 lenses. (a larger pro body DSLR would be pretty tight in the Union street and you can forget about it in the Billingham Hadley Pro.)
The real consideration for the bag is do you need to carry a laptop around with you for post processing on sight. if so then steer clear of the Hadley
and just buy the ONA. The Billing Ham is really all about being on the go, carrying a lightweight kit around a city and snapping shots. A real street photographers bag.
At over $200.00 USD both bags are not competing with the ballistic Nylon “Black (steal me im a camera bag) Backpacks” made by every other camera bag company out there. these are about uncompromising style and function wrapped into one package.
One interesting side note: The billingham bag has a removable insert. allowing you to remove the “camera bag padding” and just use the bag as a carry on or a carry all. I have found this a great way to do air travel with just a roll aboard and a hand bag. you can leave your camera and lenses in the padded insert and place the insert (with your expensive gear) in your roll aboard. Then use the Hadley as your hand bag to carry magazines, water bottles, your iPad, headphones or whatever else you want to carry on. Once you get where you are traveling to you can put your bits and bobs in your roll aboard and your camera insert (and camera) back in your Hadley Pro.
Well either way you go you are sure to be happy with your choice!