Now your talking...

I sold all of the cast iron table band saws I had when I got a late one with the bigger aluminum table but I don't consider the bigger aluminum table a big deal. With it you lose the cross slot and using the miter-gauge as a fence.
All other factors being equal like condition etc. I would probably go for the 510. I do consider the 510 table system a big deal. I have no experience with a 520 but looked at them. they look pretty nice but - "for my use" - I couldn't see enough advantage to spend to upgrade to it.
Some things I would not let influence me much such as a tired cord or less than perfect paint. The cord is a wear out/age out item. Now if there were a breakage of a casting or the like I would want to look pretty close.