I think a real deciding factor would be if you were interested in reading library books (PDF or ePub) files on your reader. Since these are DRM'ed you can't convert them for use on the 500 without removing the DRM.
I get a lot of value out of being able to read library books on my 505--even have paid $100 for a non-resident NYPL card because of their big selection.
Also, from all accounts the screen is better on the 505, so this is a factor to consider as well.
Also, how is the battery holding up in your 500? By getting a new 505, you'd get a new battery. Definitely not a reason to upgrade in and of itself, but it could be a factor.
dordale