[apendleton]

No, I think it's that even if each channel gets the same bitrate, if there's more "going on" in one channel than the other, there may be fewer bits to go around and that channel may have more qualitative loss. So if they both have the vocals equally, but one also has bass drum, the one with the base drum may lose more fidelity on the vocals than the channel with vocals alone.

[mikeash]

Excellent point! I hadn't thought of that. It strikes me, then, that Joint Stereo would actually be better in this case, as it effectively encodes the mono signal and the L/R difference, which is what you're after anyway for this purpose.