Depends on where/ what you are diving. If you are diving cold salt water w/ thick undergarments, you may want to stick w/ steel for the ballast. Anything else, you will likely want an AL plate for steel doubles. Also, if you have specific trim weighting needs, AL allows you to move more weight around without overweighting yourself. I started w/ steel with my first singles rig, then picked up an AL plate about 6 months after I went to doubles, and now I use both depending on what/ where I am diving;
AL singles- Steel BP
St dingles- AL BP
AL doubles- Steel BP
St doubles in fresh water (warm or cold)- AL BP (+ trim weights)
St doubles in warm salt water- AL BP
St doubles in cold salt water- Steel BP
These are used in combination w/ appropriate thermal protection (wet and dry). With each of these configurations, I don't need ditchable weight. It will take some time to figure out exactly what you need, so the best thing to do is spend some time w/ trial and error. AL is probably more flexible since you can always throw on a weight belt if you need the extra ballast, but not as easy to get overweighted as with steel.