Just recently got a (blue ;-) ) 350 to replace my still excellent 505 - also in blue.....
The guy who brought it over from Holland for me (if you too like the Sony blue, as i do, do, do) also got his wife a black 650 - so i had the opportunity to see and compare all 3... and all 3 with a pdf formatted in fontin font and to match the 6" or 5" screen as needed.
First - the 'old' 505 is wonderful.
But, it's best not to see anyone's new screen Sony (350, 650, 950) as the difference is very noticeable (perhaps more so with a serif Fontin font?), so...
350 is very good indeed - just switching over to epub, and looks like a fractional decrease in font size ( 1/2 a point?) makes no difference in ease of reading comparing the 650 to the 350, yet means you lose only maybe about 4 lines and a word per line per page... making the 350 very, very portable yet still an excellent ereader.
This may sound like i'm saying the 350 is the one to go for... but, then consider that due to the better screen you could possibly go slightly smaller font on the 650 - and the unit is smaller than the 505 already. Sorry to muddy the waters... good news is they're both good, so you can't really make the wrong decision.
As I had my 505 as a backup, I went ahead and Baroda'd my new 350 (!)... somewhat nervous experience (i'm in thailand so getting hold of a replacement would be a pain) but went fine and Fontin looks great & justified on epubs on these Sonys - much better than the built in 'font'.
Only real issue is the ridiculous font size increase from S to M - with 6 sizes to setup, whoever 'designed' this point increase at Sony needs to be... ah, what does this nincompoop deserve. How? How, in all that is possible, can anyone with functioning eyesight decide that this kind of step change is needed - and why not make it a user choice like in any basic word processor... ah, the 'choice' (lack of) of fonts, and the 'software' - hah. All praise Goyal, praise him.
Tried taking pics comparing all 3 - here's one of 350 then 505 then 650 (the 350 is resting on the 505's cover ;-) - the picture quality doesn't even remotely reflect how good the screens look in person, but it'll give you an idea of size comparison and you can see the number of lines of text etc.
This one with 6" PDF's for the 505 & 650, and a remade pdf to 5" screen
This one is an epub on the 350 (same Fontin font) but slightly smaller font - think you can see the approximately 4 fewer lines on the 350 cf 505.
Coops