I have a 505 and just picked up a 300. (Detailed review is in the News & Commentary forum!) I went with the 300 because I love the portability (after taking my 505 all over the place for 1.5 years). The screens and response are the same, and the 300 has almost exactly 2/3 of the amount of text per screen as the 505 (fonts are the same size, and there is 5/6 of the characters per line, 5/6 of the text lines per screen).
I love the 505. But I'm a guy, and don't have a purse/bag for the 505. I do carry it in the big thigh pocket of my cargo pants, but only in warm weather. The 300 can slip in to any pants pocket pretty nicely. (I wouldn't carry it in a rear pocket, but it feels much sturdier than the 505, being more compact and slightly thicker.)
I never used mp3 playback or memory cards on the 505, and don't need them (nor a touchscreen/dictionary/notes), which is why I went with the 300. It is basically an excellent, portable e-ink device for reading lots of books. I have over 100 novels loaded on my 300, and have about 360 out of 443 MB of memory still free! Although I've accumulated many e-books, I think there is PLENTY of memory to carry around over a years' worth of reading, and then some. I don't feel any need to carry every e-book I have with me at all times. (That only leads to more pages of menus to trudge through, slower loading times, etc.)
Just my opinion/experience. I also spent a few hours with my 300, finishing a book I started on my 505 (and then some). After about 10 minutes, the Reader details disappeared and I was just reading.
