I'm not a big fan of aluminum directly on the steel shafts. I've seen too many with pretty substantial wear on the shafts from the aluminum galling under increased spring loads. The bronze bushing is much more forgiving in my opinion, although it can still happen if the bronze is pretty hard or the oil supply isn't adequate.
I've been told by a large manufacturer of valvetrain parts that a bronze bushing in an aluminum body does not retain well. Apparently, the increased distortion of the aluminum rocker under load, as well as the more malleable properties of the bushing, allows them to work loose over time. I can't confirm this from personal experience though.
Let's face it, aluminum rockers have a fatigue life that is much shorter than a steel rocker. Their place is on a regularly maintenanced or inspected engine, and that applies to the roller bearings as well. They are pretty much mandatory on engine with high spring loads, but that type of application also requires frequent maintenance. I prefer to use a bushed steel rocker for most anything less than a dedicated race engine.
