halocline
Contributor
In selecting between AL and SS backplate one has to consider the weight. Is it considered bad practice to dive with a SS backplate if that means you have no user-releasable weights? I know a buoyant ascent is bad, but it always nice to have that option.
Ditchable weights are for the surface primarily, in the case of a panicked/exhausted diver who cannot stay afloat otherwise. There is almost no conceivable situation at depth in which you would want to ditch weights. There is a valid argument that quick release weight systems add risk, because if they fail and you inadvertently drop weights at depth, guess where you're headed? So, no, it's not "always nice" to have the option of a buoyant ascent. OTOH, there are many dive accidents and problems at the surface, hence PADI's insistence on ditchable weight. The choice is yours; I typically do my warm water diving with just a steel plate for ballast, and I really enjoy that.
You should choose the plate material based on your ballast needs and trim considerations given the exposure suit and tanks you will be diving with. Dive gear should really be chosen on it's performance while diving.....unfortunately it's too common that the sales pitch, dive shop appeal, and clever but useless features win out.