The role of the physical properties of multiwall carbon nanotubes on the strain-sensing piezoresistive behavior of multiwall carbon nanotube/polymer composites is systematically studied using three types of multiwall carbon nanotubes as fillers of a brittle thermosetting (vinyl ester) and a tough thermoplastic (polypropylene) polymers under quasi-static tensile loading. Two of the three multiwall carbon nanotubes investigated have similar length, aspect ratio, structural ordering, and surface area, while the third group contains longer multiwall carbon nanotubes with higher structural ordering. The results indicate that longer multiwall carbon nanotubes with higher structural ordering yield higher piezoresistive sensitivity, and therefore are better suited as sensors of elastic and plastic strains of polymer composites. The highest gage factor achieved was approximately 24 and corresponded to the plastic zone of multiwall carbon nanotube/polypropylene composites with the longest nanotubes.