By my understanding, both systems use gyros and accelerometers to provide aircraft's attitude and heading information. Given that the information they produce are more or less the same, why do the modern aircraft have them both? What are major functional differences of the two systems?

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    $\begingroup$ What an AHRS can do is a subset of what an INS can do. Both will give you attitude & heading; the INS also gives you position and velocity, which the AHRS doesn't. An INS is considerably more expensive as a result, but allows you to continue to navigate through losses of GPS signal. An AHRS is a solid-state replacement for mechanical gyros, but it doesn't keep track of your location. $\endgroup$ – Ralph J Oct 5 '17 at 14:50