If you are using a coincident setup (two microphones that are not spaced apart, but merely aimed in different directions for left and right), supercardioids or hypercardioids will generally give a more evenly distributed stereo image than cardioids, which tend to bunch everything in toward the center in most coincident setups.
It's important to check the frequency response curve of any super- or hypercardioid microphone that you are considering buying for music recording. Some manufacturers assume that highly directional microphones will be used for dialog recording (film and video sound) rather than music applications, so they roll off (reduce) the bass response of the microphones more than you would probably want.
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