Why bother with all that weight when you can fill an 85 to 4k and have a lightweight 120
As others have said, the decision between using steel and AL cylinders comes down to the dive. Most dives I use steel for my primary cylinders and AL for stages and/or deco bottles. For sumps I will use whatever I can get someone to help me carry to the water; that may be AL 40's or lp 45' or even lp 72's or perhaps AL 80's or lp 85's. It all depends on what is being planned and the conditions of getting to and from the water's edge.