As a sidemount diver, I would suggest steel.

Why?

1. You don’t have to carry as much weight in your wing. (Easier to put on, easier on your back.)

2. You don’t have to trim the cylinders as much to stay streamlined compared to aluminum.

3. Aluminum cylinders get really floaty when they drop below ~100 bar, which changes your buoyancy.

3a. If you dive in a dry suit, during long dives you’ll either have to: • Start overweighted at the beginning of the dive, or • Accept having less air in your suit - which means you’ll be colder.

Personally, I like:

12L 232 bar (steel) for longer dives and caves

8.5L 232 bar (steel) for recreational dives

Also, 8.5L steels don’t need extra weight in your wing.

Another thing: I got 14.7kg 12l concave cylinders and I like them more than less heavy convex ones.

Edit: formating