In this case, you're overthinking it quite a bit. I've used a 6700 rd with 6603 shifters, and my experience matches the conventional wisdom that they are 100% compatible. (Brakes are a slightly different story, but still compatible for all practical purposes .) On the other hand, you're very unlikely to notice a performance difference between them. I'd get whichever is cheaper or whichever you think looks nicer.