I am assuming you are talking about solid and stranded UTP. The solid wire is what is normally used for building wiring, while the flexible (stranded wire) is only used for patch cables. The signal attenuation loss in patch cable is considerably higher than the solid wire, and according to the EIA/TIA standards there are limitations on the length of patch cables used because of it.
Stranded wire will not properly terminate on IDC (insulationn displacement connector) jacks and patch racks. These are the kind that require a punch down tool to terminate the cable. Solid wire should be used to terminate on these jacks. Also, if you are putting modular plugs on the ends of the wire, there are different types for solid and stranded wire, and using the wrong one can be problematic for the connection.
Hope that helps.
It is only my opinion, based on my experience and education...I am always willing to learn, educate me!
Daron J. Wilson, RCDD
daron.wilson@lhmorris.com