The aluminum is probably more susceptible to denting, being a softer metal.
Will you ever need to walk on the roof? Do you have overhanging trees that may drop branches in a storm? Even trees near your house can see branches go flying your way.
Another question is the coating used. Since steel can rust, you don't want to worry about it. A high quality product would seem valuable there. Of course, you don't want to see paint flakes peeling off an aluminum roofing product either.
Finally, make sure that you can deal with snow, although this is not a factor that depends on the material itself. Our neighbor has a metal roof. A few years ago, we had a large, heavy snow fall, and a huge mass of snow broke free to slide off the roof. As it was coming down, it tore off the gutters on their home, then completely flattened part of their porch. My guess is it was several thousand dollars worth of damage. Falling snow can also damage plantings near the house. There are tabs that can be placed on the roof surface to prevent this if you live in the snow belt.
The three things that I ask people to consider when they're replacing a roof is if they're in a wind or hail damage region, and if they're in a region with high solar gain (such as Texas) or an area prone to ice damming (such as New England or the upper midwest.)
If you're in an area that is prone to wind or hail damage (thunderstorms or hurricanes), you might want to look into a more resilient form of roofing material. Stone-coated steel shingles (which look just like standard asphalt shingles) are a great product that will resist most forms of damage that can be dished out. Your insurance may give you a discount on them because they have a longer lifetime.
If you're in a high solar gain area, look into some of the energy-star rated shingles. They again look just like stone coated asphalt shingles, but they'll help keep your attic cool in the summertime.
If you're in an area that gets a decent amount of snow, make sure that they install an ice & water barrier around any roof penetrations (like skylights or places where they replace a vent) and along the edge of the roof where you could develop ice damming problems. This material is a sticky membrane that self-sticks to the roof deck and basically keeps water from seeping through at all. Common brand name for this product are "Grace Ice & Water Shield" ... I also personally use it in roof valleys, gable crotches, and several other places where water tends to get blown up inside something and you really don't want it to soak through.
You definitely want to get your insulation up to the max, but that's something that you can do at any time using blown-in insulation. If you are replacing roof decking and have it open, you should make sure that you have plenty of soffit vents and that your soffit vents are baffled properly. (I like the Berger Accuvent, personally...)
You want to make sure that any damaged or rusted flashing is replaced, and that tar paper is replaced. Look into what kind of tar paper they're using (heavier the weight, the better...), because that's actually your roof. Make sure you know what kind of valleys you're getting. For standard asphalt shingles, I prefer woven valleys, but they're harder to do and therefore are more expensive.
Best Answer
There is a maximum exposure for wood shingles. 18" shingles will have the same amount, whether tapered sawn handsplits or perfections. Maximum exposure is based on the remainder of the exposed part of the shingle to extend up under 2 more courses that way, where the nails are that hold the shingle in place, which by the way is always 2 nails regardless of how wide the shingle is. That way the water passing around the edges of the shingle onto the shingle under it has another shingle to get around before it causes a leak. The other guide line is that the shingle gaps are to be no closer than 2" apart from the shingle above or below.
On a 4/12 pitch, the rule may change there you may need to tighten up the exposure where the shingle top is covered by 3 shingles and not 2.
For example, maximum exposure for an 18" shingle will be 8" for a roof with a higher pitch than 4/12. On a 4/12 pitch the exposure would need to drop to 5 1/2".
With a lower pitched roof the rain water will not move off fast and would have a chance to migrate deeper around the shingles, therefore more chance of a leak.
The butt thickness only adds to the longevity of the shingle.