40 plus years in the electrical distribution industry.
There is no difference in the wire it self. Both are manufactured to AWG standards. The difference would be in the anealing of the copper and the number of individual strands in the conductor. The portable cord will have more strands and be softer(more flexible) than standard automotive wire. The insulation will also be a rubber compound material.
In short 14 gauge wire will carry the same current regardless if it is portable cord, automotive, or building wire.