A library often implies a specific foundry, and a given foundry may have limited package selection. Packages also vary in the capacitive and inductive loads that they present to the rest of the chips in the system. It is important to have a power budget in mind from the beginning. The power budget affects the cost by its impact on package style, cooling methodologies, and general system and environmental considerations. Thermal dissipation aside, the interests of power and speed are diametrically opposed.
Power budgets can even eliminate certain libraries from the outset.
Product Life Cycle One important aspect of choosing a library is the intended product life cycle. What may be an attractive solution from a cost standpoint today may be obsolete and difficult to find in a few years. You may want to consider high-performance libraries, not because you need the performance, but because they will likely be the standard during the life of your product. Pushing the limits may require that you participate in shaking out the bugs of a new process and, at times, take on development risk. In a project with a very long development cycle, however, consideration should be given to new libraries which will be mature by the time you are ready to proceed to silicon.