Does a Skylight need to be installed with the dedicated flashing system

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.

Its odd looking, but the batten support lets the tiles 'notch' into place. You'll see the same support for clay tiles, also.

Because of the 'hanging' aspect of the tile, most locales only allow tile roofs for pitches between 4/12 and 6/12 (rise over run). Lower pitches are possible, but then waterproof membranes are required. In those cases, sheathing support for the membrane would be needed.

You don't mention it, but not using sheathing is usually only done in warmer climates, where summer heat is the main problem.