Easily the best bullet point for the C500 over Red is that you have a crazy amount of options over what you want to shoot in, from 1080p 4:2:2 50Mbps XF to 4k raw (and options in-between those two). With Red, you get Redcode, and then 1/2 debayered SDI output for anything else.
Red, however, gives you more framerate options, including the ability to do HFR, which the C500 won’t do.
C500 will be a bit more compact setup depending on what you get for external recording. You also give up the handheld cability of the C300 because the grip is replaced with more connectors.