As Andy said; it really comes down to how much weight you're carrying. I happen to have both packs. The REI one is fairly new to me so I need a bit more trail time with it to confirm my initial feeling; but I will probably end up selling the Jam.
They're both great packs. The Jam is well designed but has a lower total volume and is frameless. The REI seems to carry more weight with less discomfort (but as always, YMMV). As a point of comparison, my pack weight including food, water, and camera gear tends to hover between 20 and 25 pounds. That weight at the end of the day tends to make my neck and shoulders sore with the Jam.
My thoughts are that if you're carrying less than 20 pounds, either pack should work equally well. More than 20 and I lean to the Flash 62.