For stage bottles AL80s always.

Sometimes for deco S72s are nice for cause they stay on the bottom, have a lot of gas and don't float even when empty but AL40s are generally more convienient. Especially off of boats deco bottles of preference are AL40s because you have to lift them on and off the boat without getting killed or bending yourself. Often you may be carrying your deco bottles with you the whole dive and 40s are almost unnoticable compared to 80s.

Those steel 50s are a nice blend of characteristics - and hold a little more then AL40 when you need that little extra. Generally preferable to a 72 but AL80s are still better for stage diving. They cost a lot more then AL40s I think but rarely will you see all that much benefit compared to AL40s.

If you were getting 2 bottles for extended staged deco (for like Trimix) I'd might get either 2 AL40s or one AL40 and one S50. Since I have a S72 around the house for the rare occasions where a little extra is needed I'd rather have two AL40s - and a AL 80 often works well for that also.