Regardless of which type of alternate air source you select, remember that its performance will depend on the ability of the regulator first stage to deliver air, particularly when it is supplying two second stages and a power inflator. The bottom lines: Given the type of diving you plan to do, how will it work for you in a worst-case scenario? How much risk are you willing to accept? If you dive solo, under demanding conditions or in overhead environments, the greater your need for a completely independent air source and a high-performance regulator as your alternate air source.
(Excerpt used from New Diver Magazine)