I might start with why I would not buy 5" but love 6".
The amount of text that fits in one line. I hate it when there are 2 words per line. It breaks my concentration and ruins reading enjoyment. On very rare occasions I read in Large fonts on 6" (when on a train/bus), otherwise small or medium. 5"? Out of question. However, 5" is at least smaller and I see its benefits.
Sony 7". It is bulkier than 6". It would not fit my DVD tin box, it is heavier and generally less comfortable because it is tall and its mass centre is not located as comfortable as in 6". I would avert my eyes from all of these drawbacks, more over, I would probably not notice them if the difference between 6" vs. 7" would be similar to 5" vs. 6". Bigger screen, wider text line........oops
We get the same width! The screen is only taller, but not wider. I don't care about its hight. 6"'s hight is fine by me. I want the screen to be wider, to grow in size proportionally. Like iRex iLiad's 8". Drawbacks entailed by handling a bigger ebook reader should be cancelled by some benefits. The only benefit for me would be wider screen. Like 5" vs. 6". Otherwise I would buy 5", because the raw amount of text per screen doesn't bother me.
What do I get from Sony? A bigger reader, heavier, less portable....and the same screen size (from my point of view)? Not interested at all.
P.S. please do not suggest landscape mode. I don't like holding reader like that.