The Neumann will certainly be more accurate than the Shure. Many people prefer small condenser mics for
acoustic guitar such as the Shure SM81, or NT5 (matched pair for stereo micing), Neumann KM183 or 184, Octavas etc. The SM57 will have a greater proximity effect, so at a close range, it may be more difficult to avoid too much bass without significant filtering and/or EQ on the preamp.
The best placement for mono recording? A good starting point is near the 12th fret about 12 inches away, angled slightly towards the neck. The trick is to get a good balnce between the higher and lower frequencies. The closer to the sound hole on the guitar, the bassier the tone. But the best thing to do is to use your ears and experiment. Your room comes into play somewhat as well as the guitar and guitarist. If the guitar is to be featured on the recording, go for a full sound. If it is to be placed as, say, a "jangley" rhythm track, go for a slightly thinner sound, emphasizing high mids and trebble; it'll lay in the mix better.
But there's nothing to stop you from playing around with the SM57. You might get some interesting results, depending on what you're going after.
