This paper deals with the comaprison of 3-RPS and 3-SPR parallel manipulators based on their operation modes and singularity-free workspace. The operation modes of the 3-SPR ma-nipulator are identified by using Algebraic Geometry. Those operation modes amount to the operation modes of the 3-RPS parallel manipulator, which has already been studied in the literature [1]. Then, the parallel singularities of the 3-SPR and 3-RPS parallel manipulators are analyzed in order to trace their singularity loci in the orientation workspace. An index, named Maximum Inscribed Circle Radius (MICR), is defined to compare the two manipulators under study. It is based on their maximum singularity-free workspace and the ratio between their circum-radius of the moving-platform to that of the base.