The steel 85s will only give you 5 cu ft more air than the aluminum 80s if both tanks are filled to their rated limit so no real issues as far as diving together regarding length of the dive. Some of the advantages steel offers are longevity (if cared for properly), you’ll have to carry less weight to adjust your buoyancy (partially replaced by the weight of the tank) and smaller size relative to an aluminum tank. If you haven’t bought yet, check into getting 100s. They’ll give you the extra capacity should you need to buddy breathe will someone on an 80.