I haven't exactly done a scientific analysis, but I can tell you what I've observed as a recording engineer and player of both top types. The differences seem to be in frequency reponse and saturaration characteristics.
FREQUENCY RESPONSE: The cedar top actually seems to attenuate some of the upper-midrange frequencies when compare to a spruce top. However, the high frequencies seem to be intact, which give the guitar a rounder sound but a very crisp high end.
DYNAMICS AND SATURATION: A cedar top saturates entirely differently from a spruce top. Because of its stiffness, a spruce top requires a fair amount of string motion to get moving but it will also begin "compressing" early on when it is pushed with heavily-strummed chords. It simply limits the increase in output for a given increase in playing pressure. There'll also be an initial bloom of high freqencies (even-order harmonics) as the top compresses, which gives a nice, bright "chirp" to the sound. The compression occurs over a wide loudness band so you rarey really saturate the top. Because of its greater flexibility, the cedar top reacts to light playing with much more volume than the spruce does, making it more "sensitive" to nuances but it compresses earlier and over a very narrow loudness band before it suddenly saturates with very little warning. When it reaches saturation, it has very little harmonic bloom and instead seems to begin putting out non-harmonic overtones (odd-order) which give it that jangly sound.
Where does that leave us? With two different guitar tops for two different uses! The spruce yields that nice strumming sound we all love but is much more of a bear to play fingerstyle on. Its single string definition isn't as good and the individual notes don't seem to have as much body as those from the cedar. The cedar allows a much lighter playing style for fingerstyle with a wonderful, wide response band up to its saturation point. The loss of the upper-mids actually makes individual strings sound full. The trade-off is that when you overload it, it turns to hash and sets your teeth on edge.
Play both fingerstyle and strumming when checking out a new guitar in order to get a full idea of what the guitar can do. Viva la difference!