There's really no difference between the two other than the way they look and physical button configuration. Some people say the 500 series is worse than 600 for quality. I've heard good and bad stories for both remotes, overall I think they are about the same but have no statistics to prove which is better... I've heard as many or more bad stories about the Harmony 880. The 600 series is based on an older design and so is unable to replicate some high bit rate IR commands used for certain newer devices, like some PVRs. The 360 is basically a 500 series remote with the coloured xbox buttons.

You might want to save up and get the Harmony One, it's the best one out now.