I have a 505. After reading quite a few comments and comparisons of the kindle vs the 505, I'm very glad to have the 505.
It seems to me, just from posts on this forum, that the 505 tends to be sturdier and more durable.
Since I read library books almost exclusively (although since I can get a lot of free books at places like this forum and feedbooks and such, that works too!), I love the ease of downloading library books onto my reader and returning them early and getting more.
Since I really just want to read, read, and read

I am glad I have something that doesn't have any extra bells and whistles, and just allows me to read read read! I find I really don't need to "buy on the go." It's just as easy for me to upload books (and charge it on the computer at the same time!) once every few days or so and then I have as many in there as I want. RIght now I have about 86 on my reader, about 76 of which I have not yet read. I doubt I'll need to upload any on the go.
From what you post, I think a 505 is perfect for you, but I could be a bit biased....