Screen on PRS-600 is terrible (touchscreen layer corrupts it) so I wouldn't even consider it. (low constrast, reflections).
Also note that PDF (unlike most of the other formats used on ereaders) format is basically a set of instructions describing how to print a page, you can zoom a page, but not reliably split into pages of different size, except when it's splitting existing page into 2, maybe cutting margins. (that's why some PDF => ? converters basically treat it as a picture that needs to be OCR-ed)
Imagine how A4 page (it's size close to US Letter) looks on a screen slightly bigger than A6 (6"):
In your case I'd opt for the reader with the biggest screen and highest contrast - the 950.