I was at my LBS today for something else, and I asked them about this. They understood what I was asking, but didn't seem to think it to be significant. They did agree that the SRAM concept is a little easier to adjust, as the derailer doesn't "waggle" on the pivot -- it stays pretty solid. I looked at a new bike with a SRAM X7 RD, and it was the same concept as the 3.0 RD on my wife's hybrid -- that B-screw threads directly through the housing at the mounting pivot, and rotating the RD clockwise about its mount lifts that B-screw up off the hanger. Letting the RD rest against the B-screw, it cannot rotate forward and further.

I thought this difference is more notable than it apparently is! :-)