As the every development team we have to survive the big stress in the beginning of the project: our clients usually have a lot of ideas, requirements and push us to give them the estimations ASAP) Once we tried story mapping as the requirements gathering tool and sticked to it for a long time. Story mapping appeared to be a great thing when you want to understand what your system is intended to do.
Prioritized backlog helps a lot, when you want to know what to do next, but it is not the best choice, when you want to think about the whole system. A user story map arranges user stories into a useful model to help understand the functionality of the system, identify holes and omissions in your backlog. Also it is a great tool for planning releases.