K, there's a technical disadvantage to flat wire which is largely of just theoretical significance. The inductance of speaker wires varies directly with the distance between the conductors and flat wire has the conductors separated more widely than standard lamp cord, resulting in higher inductance. This causes slightly greater loss of the very highest audio frequencies, but except on a very long run the difference wouldn't be audible to even the most golden of ears.
So, use the flat wire of your choice if that's the most convenient for your setup; it won't audibly affect the sound.
