I watched the video for the Ninja bag. I personally would not buy it. The "attachment mechanism" is simply 2 U brackets. I've built a bag like this before. Those U brackets are not enough to keep the bag from bouncing off of the bike, even if you hold it down with a bungie.

Go with the Arkel. They know how to make panniers that'll never fall off of your bike. $170 for a good backpack isn't a bad deal. Look on amazon or ebay, you can probably find it cheaper.