I've not used the Stamina but the Rocketpak was poor compared to some of the other options available. The material it's made of is thinner, build quality not great and generally uncomfortable to wear. I had to sell mine in the end at a loss. The Stamina although looking the part seems to be of similar spec.
IMO I would spend the extra bucks and get the Sabre 45 or the Berghaus Centurio.