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.

Less chance of cracking

Standard shingles are a bit thinner than ridge cap shingles, and can crack or tear when folded over the ridge. Ridge cap shingles are thicker and come pre-bent, so there is less chance of cracking.

Better adhesion

Most ridge cap shingles have an extra dollop of adhesive near the end of the shingle, to help hold it to the shingle below. This helps to prevent blow off in heavy winds.

Thicker nailing surface

Ridge cap shingles have a thicker nailing area, which helps prevent tearing near nail holes.

No extra prep

Since ridge cap shingles are designed specifically for the ridge, there is not preparation required. Just open the box, and start slapping them on.