While energy storage technologies have existed for decades, fast-ramping grid-level storage is still an immature industry and is experiencing relatively rapid improvements in performance and cost across a variety of technologies. In this innovation cycle, it is important to determine which properties of emerging energy storage technologies are most valuable. Decreased capital cost, increased power capability, and increased efficiency all would improve the value of an energy storage technology and each has cost implications that vary by application, but there has not yet been an investigation of the marginal rate of technical substitution between storage properties. We use engineering-economic models of four emerging fast-ramping energy storage technologies and examine their cost-effectiveness for four realistic current applications. We determine which properties have the greatest effect on cost-of-service by performing an extended sensitivity analysis on the storage properties for combinations of application and storage type. We find that capital cost of storage is consistently important, and identify applications for which power/energy limitations are important. Each combination is different and blanket statements are not always appropriate.