As others have noted, the red frame steers a bit quicker, but I didn't feel like it sacrificed stability. I assume the quicker steering is due to three factors:
Shorter wheelbase Narrower rear tire No steering damper. The Bitubo damper that came with the greenie had a noticeable sticking point, so I removed it.
Other differences between frames for the record:
Front subframe - the black front subframe is beefier and has more cross bracing. I assume that this would be interchangable with a red frame; the distance between the two frame-mounting points is the same.
Lower (center) subframe - as noted elsewhere, the black lower subframe has additional cross-bracing, including a significant one to the LH porkchop (which makes clutch slave cylinder access extremely difficult on the black-frame bikes). It also has arms that go around the transmission to the engine. The black ones cannot be bolted directly on the red-frame bikes, because the engine case would need to have holes drilled in it - and there is a small difference in the LH porkchop mount area of the spine frame where the cross brace attaches.
Rear subframe - the black frame has tabs to mount the rear fender/light assembly. The painted tail sections of the red-frame bikes are heavier and mount differently than the tails on the black-frames.
Shock reservoir bracket - I noticed that these are different between the two frame types. It's because the bracket mounts over a brace-point on the black frame bikes, while there is no brace attached behind the porkchop on the red frame bikes.
Transmission Brace - The red frames have a bracket welded underneath, just above the place where the engine and transmission cases join. It seems that some early models had a brace there, but they stopped installing the braces at some point. It also seems that without this brace, the engine and transmission may move too much, putting pressure on the whole system, and could result in a crack in the rear of the transmission case.
Edit to include trans brace from discussion below.