The 500CM has a shutter status indicator, which I personally happen to like and I'm sorry my 501CM doesn't have this. I can live with my camera easily as it is but I'm thinking of getting a 500CM as a second body in the near future. Since the longest lens I have is the 150mm, vignetting is not an issue.

So, in my humble opinion; getting a 500CM in good nick would be a good idea.