Good Info
The first two responses answered all pros and cons accurately (imo) the only thing I can add is that in regards to the Cetme, both mag types functioned equally reliably in my rifle. The steel mags are in my understanding, Cetme mags usually available for $10 or less, 5 round mags, making this type rifle legal to hunt with and a lot easier to shoot from the bench, are also available.
The aluminum mags are typically German milsurp, usually available for $1 or less, I have seen them as inexpensive as $50 per 100. My personal preferance is for the aluminum, they are much lighter and do not rust.
As a side note, if you are considering an optic for your rifle, there are other options besides the Stanag style mounts/rings that are every bit as stable, and far less expensive, including picatinny options, as much as I liked my Cetme, I appreciated it even more after I mounted a scope on it.