Neither wood is best for everything, and what either one is good for depends on a lot of things, like scale length, bridge type, neck and fretboard woods, the type of pickups you use, the amp you abuse, and most importantly, the style of playing you have.
For instance if you have a more rock oriented sound, a 24.75" scale, mahogany guitar will sound great.
That being said, the difference between mahogany and alder in identically designed guitars is this:
Alder will resonate a wider frequency range than mahogany. More mids and low mids are present in alder's sound, whereas mahogany has more of a notched mid that creates more apparent bass and a slightly brassy treble over the top. Mahogany is rightfully declared a "warm" sounding wood because it's scooped frequencies make it sound woody, especially in the lower registers. Higher registers of a mahogany guitar take on a somewhat tinny tone to my ears. It seems like more of the fret-on-string sound is amplified by mahogany. Additionally, because mahogany is a hardwood, it compresses the tone of a guitar. That's why mahogany guitars seem to have so much sustain. But that sustain comes with a price. Some mahogany guitars are made of a very thick slab (I'm specifically referring to LP's) that is meant to resonate for a loooong time. This can be a good thing if you play doom metal, but if you play anything with fast, staccato runs, or start/stop rhythms, you may dislike the woof you get at the end of every palm mute. I personally hate it. The Gibson's I've tried are frequently like that. Thinner mahogany bodies don't seem to have this problem.
Alder on the other hand is quirky too. Alder strats (with a pick guard) tend to sound nasal in a metal setting. Think of the sound of Nile, for example. I used an alder strat for high gain metal for a while, and while it rewarded chord complexity, it was punishingly sloppy sounding when playing fast palm-muted runs...although this can be compensated with the right pickups. On the other hand, alder guitars that are rear routed sound quite different. Alder shouts where mahogany would have purred. If you like shrieking, vocal pinch harmonics, there are few woods better than alder. Alder also has an apparently thinner, less compressed sound than mahogany. This is bad if you want an ultra-thick, gooey ooey, sludge-metal tone, but great if you want precision leads and play odd chordings. Alder has a slightly wild harmonic quality as well. It can be a tad shrieking at times.
I like to break the qualities of the woods into a few categories:
Mahogany really thickens up single note palm mutes, but can get a little sluggish if the wood is overly resonant.
Alder isn't as chuga chuga sounding, but has a more balanced attack and decay.
Mahogany has an understated clean tone and to my ears has a kind of Lynrd Skynrd sound to it clean. Clean neck tones are very chimey and smooth.
Alder, because it accentuates a lot of mid frequencies and some highs sounds a little twangy in clean settings on the bridge, but sounds very good on neck pups.
I might step on some toes here, but I think mahogany is a bit too nasal for most soloing (unless you're playing an LP). Mahogany makes leads sound pinched to me.
Conversely, alder gives a lot of clarity, bite, and a singing quality in soloing. Additionally, tapping is more balanced from string to string than mahogany.
Mahogany is more dynamic to me. You can go from gentle to crushing and it is quite expressive.
Alder though is brasher and doesn't respond as well to nuance. However, alder sounds better in a situation where you are really laying down the law with a heavy palm muted riff.--Mahogany tends to flatten out a bit in this area.
Final analysis: Both are very good for metal, but I'd say go for mahogany only if you're doing more atmospheric, dynamic music, and if balls to the wall is what you want, I'd vote for alder. Better yet...just get both. You wouldn't paint a fresco with one style of brush only, would you?
Last edited by Resonance : 2009-04-16 at 03:14.