Bach and Chopin each have their advantages over each other, thus they are in the same position. Bach trains you on finger independence, polyphony in each hand and evenness. Chopin trains you on everything else, but not necessarily what Bach trains you on. Sure, the Chopin etudes need finger independence, polyphony in each hand and evenness, but they don't train you in those areas, except polyphony in some pieces. They simply require them to be played well, but they are not needed to simply be played. Sure Bach is in the same position, not necessarily requiring them to simply be played, but every piece of music ever wrote doesn't either. A 5year old could play Bach and Chopin, but not necessarily with finger independence, evenness, and crazy octaves.
Basically, Bach is better than Chopin etudes for learning finger independence and evenness as they are more a requirement of his music than they are of Chopin's.
Henrah