IDK, but, if you're not commercially cutting every single day, it matters little. Heck I haven't held or ran the 501, so my opinion is of little value. I'll offer this, perhaps spmeone esle will chime in wiht the exact differences, ask them if the average user will actually notice the differences? I have ten saws I use, and I am not into modding any of them for any reason. I used to muff mod and mildly port, once a machine needed opened up for whatever reason, but for a hobby? Not worth the time any more. If I need more power I grab the bigger saw. I can switch a saw for the log or tree at hand. If you are looking at one primary, the 60 cc 590 is a good relaible saw, if you need a bigger saw, the tree istelf is also going to be heavy to work with blocking up big rounds- its just snowballs.
For me, the reliability is king, and I can buy anything I want, and lots of them, but why be wasteful?