For applications requiring a large dynamic range, real numbers may be represented either in floating-point (FP), or in the logarithm number system (LNS). Which system is best for a given application is difficult to know in advance, because the cost and performance of LNS operators depend on the target accuracy in a highly non linear way. In doubt, designers will choose floating-point. This article demonstrates a methodology for a better informed choice thanks to FPLibrary, a freely available, dual FP/LNS arithmetic operator library. FPLibrary may be used in the prototype phase of an application to obtain, with low design effort, accurate measures of performance, cost and accuracy of both LNS and FP approaches. Two case studies demonstrate the benefits of this methodology.