Back to the point of 500 series... they are primarily used in the USA, though I wouldn't doubt they will become more popular around the world soon.
The problem with 500s vs rack is the power supply. Rack mounts have a dedicated power supply that is "tuned" to that specific piece of equipment. 500 series is more generic, which means a power supply from a different company paired with a module from a different company my not sound the best. I haven't tested this, nor do I own any 500. I've only read discussions from manufactures and boutique dealers about how they do this when I was building my own gear. My guess is if you stay with the same company for the modules and power supply, there wouldn't be any degradation of sound. I was tempted at one point to go with 500 series equipment, since most everything I do is location recording, but I can't justify the chance that the audio quality might be less. So... I haul around 300lbs of rack gear. Who needs a gym membership when you're a recording engineer?