Spruce. Although I've yet to do my own A-B testing, I'm told it sounds brighter.
Yes, spruce tends to be a bit "brighter" than mahogany. It is also generally more "balanced-sounding" than mahogany as well. This can be either a good thing, or a bad thing depending on the sound you want from your guitar. Spruce generally sounds more balanced, while mahogany has a warmer and more of a midrange-forward projection to it. Neither is "better", it all just depends on what sound you want from your guitar.

Another common top wood is cedar, which is used in a lot of classical guitars, but it is used in steel stringed acoustics as well. Cedar is noted for its warmer, darker, and sometimes fuller sound. It tends to sustain a bit less than spruce though. Again, it's all about what kind of sound you are looking for.
But then there's also the back and sides of the guitar as well. I listed the three most widely-used woods for acoustic guitar tops above, but there are probably
a hundred times as many woods, and variations of woods, that are used for the back and sides of the guitar. The three most common woods for the back and sides of acoustic guitars tend to be mahogany, rosewood, and maple, but there are TONS of other woods used as well. Again, mahogany has a certain warmth and midrange-focused projection, rosewood tends to be very "large" and "deep" sounding, with an emphasis on lows and highs, and maple tends to sound bright, clear, crisp, focused, and transparent.
But as much as the countless classic acoustic designs (rightly) rely on spruce, mahogany, and rosewood for their timeless sounds, there are all kinds of woods, and combinations of woods, that can give you beautiful, characterful, and textured acoustic sounds. For instance, I have heard cedar or mahogany topped guitars with maple back and sides that gave the best of both worlds: the warmth of a cedar or mahogany top, paired with the clear, crisp sound of maple to combine the warm and bright qualities of each wood. Really, the combinations of different sounding woods that can be paired together are staggering. Add to that the different body shapes and dimensions, as well as neck materials and bracing patterns, and the possibilities are literally endless.
Electric guitars: solid or hollow body?
Solid.
Electric guitars: Hollow body, or semi-hollow body?