I've been casting for the last 50 years and beside the valuable experience, have accumulated over 120 boolit molds made of all different metals, steel, brass and aluminium.
So what is best? I don't have one preferenced over the others.
Steel molds are great for stability and consistancy; all my precision match grade and lathe bored molds are steel. Once set to casting temperature they can turn out tens if not hundreds of boolits with a variation in weight as little +/- 0.0015 grains. The finished cast diameter is also perfectly uniform.
Brass molds are also good for precision boolts. Faster to heat to casting temperature but also by running alloy a little "hot", can gain almost 0.001" in size due to the difference in the co-efficent of thermal expansion to steel. If having a small break from casting the mold cools more quickly, so you may need to cast some dummies before beginning production once more.
Aluminium molds, such as Lee, heat up quickly but also loose heat quickly if you casting process is not consistant. With molds over 30cal, casting with a 'hot' alloy can increase the finished diameter by 0.002". A good example of this is my Lee 220gn boolit mold for the 338WM, Cast hot it throws 0.3395" boolits 'as cast', perfect for the application.