I have both the Brain Bag and the Synapse.
I bought the Brain Bag first, it is the best large size backpack on the market, hands down!
I bought a second Brain Bag and they, are, my husband and I travel bags inside the U.S. One held very heavy books for a class we took together and they have been invaluable in all our car travel within the U.S as well as move(s).
The Synapse has been designed as a small Brain Bag, people with a shorter or smaller frame than the average 5'7" person wanted them.
I wanted one to go around town.
If you look at them on the Backpack page, given my nickname you know that I have spent a lot time there, you can see that the Synapse and Brain Bag have the same configuration.
The Brain Bag has no lower pocket because it would be awkward to take stuff out of there from a full Brain Bag.
The water bottle pocket is replaced by the ingenious clip and strap setup which enable items bulkier than water bottles to be transported, such as a tripod or a packable wind breaker...
Get the Brain Bag for all your upper elementary and middle school children!
This way they won't have to endure the muscle pain and utter frustration brought on by the many other bags before I found and bought my loverly Brain Bags.