[Five]

using the mic input for mics and the line input for line-level signals is more theoretically sound as that's how the circuit was designed. Of course it is also possible to use an external preamp that is of higher quality than the internal circuitry to convert mic-level audio to line-level to give excellent results using the line in. I haven't had good results plugging a mic into the line in personally and have only seen it recommended as a desperate hack method of avoiding distortion (from high spls) at the mic input when all else fails. A battery box or some form of external phantom power is necessary to even get signal at the line input, of course.

But nobody's going to argue with results... everyone should try it both ways and see for themself which is better, sometimes the "wrong way" is the right way.