I use both, and my observation is that the red is a bit softer and stickier than the black.
Since riding conditions change so much, I don't know how you would determine the difference in effectiveness.

One thing I've noticed is that the red seems to be more prone to squealing on a humid day. Here in the Midwest, we get some very humid days in the summer, and that can cause my brakes to squeal... and these are brakes that wouldn't squeal in drier weather. A bit of hard braking helps heat the pads and drive out the moisture, stopping the squealing for a while at least. On my vintage bikes, the brakes are Campagnolo Record side pulls, and I get a bit of squealing. As always, squealing is also dependent on any play in the pivots, so proper adjustment is important.

I don't recall getting any squealing from the Weinmann centerpulls on the Raleigh International, though, and it has the red pads. The short distance from the pads to the pivot must make them quite stiff, and the plastic pivot bushings don't have much play. Or maybe I just don't ride it on the really humid days because I don't want to sweat on it??

Anyway... I don't see a huge difference between red and black.

Steve in Peoria