Today's saltwater battery is actually a fresh implementation of a 200-year old idea. The sodium-sulfate chemistry is non-toxic, non-corrosive, non-flammable, and contains no heavy metals - a refreshing change from the environmental toxins found in other batteries' cells. The saltwater battery is especially suited for long-term stationary applications like off-grid homes and work sites. They don't suffer from chronic low states-of-charge, or being operated over a narrow range of their capacity: there's no "memory" effect so there is no need to exercise if used in a grid-tie application where they may be in float mode for months on end. Aquion batteries are sealed so there's no need for venting or refilling. They're also more tolerant of high temperatures than lead-acid.
With all these benefits, it's a wonder anyone would want lead-acid! The limitations they do have, however, is they are limited in how fast they can deliver or accept energy, and that they are rather large and heavy for a given capacity. Aquions are more likely to find utility in domestic or 24-hour-runtime use rather than short-duration high-peak load applications. The banks are often sized based on peak load rather than energy requirements.