What are the iron bars and threaded rods sticking up out of the backyard

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.

A welded connection can always be made the same strength as the original steel by using a full penetration but weld but this all depends on the quality of the materials used for the welding and the quality of the welding itself. Therei s a very good reason why there is a lot of non destructive testing used when welded joints are being used for structural reasons. Other types of welds such as fillet welds will be weaker than the origianl material with the strength depending on the quality and the design.

For a bolted connection, the connection will not normally be as strong as the original material without some form of strengthening because of issues like the reduction in area caused by drilling the holes and the small lever arms caused by overlapping plates.

In this instance, if you are not intending to get the design professionally done, I would suggest a bolted connection would be more suitable unless you have absolute confidence on the quality of the welding and the design of the joints. A simple bolted connection should give you more confidence about the strength than a welded joint even though the capacity of the connection may be lower.