Thanks for the replies everyone.
I got up this morning and started poking around forums and review sites again and came across something that hadn't occurred to me before: the PRS-350 has noticeably higher PPI than the PRS-650 which should result in crisper text on the 350.
Obviously I'm not the first person to think this:
https://www.mobileread.com/forums/sho...d.php?t=115812
This really intrigues me

...and the lower price of course. I hadn't seriously been considering the 350 since I'm "convinced" it's too small, but in my research this morning I've seen comment after comment about people choosing the 350 over the 650 or the Kindle 3 and having no regrets about the smaller screen size. I know it's all subjective in the end so I'll see if I can get to a Best Buy (in the USA here) and try one out in person. I wonder if they'd let me bring my 505 in to do a side-by-side comparison.