I've just received Sony 300, unit #10. I'll add pictures in a few hours.
Size:
Opus is a bit smaller than Sony 300, as Sony is "wasting space" with its buttons. Opus reminds me of my former PDAs, whereas Sony 300 doesn't seem that much smaller than Sony 505 for example.
Design:
To me, Opus is a bit "plasticky". It doesn't look as "cheap" as BeBook, but definitely less elegant than Sony's units. Sony 300 looks similar to the former Sony units, but it's available in a broader range of colors.
Screen rotation:
Cybook Opus: Motion sensor, auto-rotation like on iPhone. Working very well.
Sony 300: Manually via menu or zoom button. Way less convenient.
Display:
Both are 5" units, so obviously the display size more or less is identical. I don't see any difference in display quality (contrast, glare, sharpness, ...) either.
Pouch/Sleeve:
Both units don't have covers included, but sleeves.
The Opus sleeve seems way more stable. I've already ordered a cover from Sony, whereas I haven't found one for Opus yet.
UPDATE: noreve.com does have a leather cover for Cybook Opus for € 50.
File formats:
Cybook Opus supports .epub and .pdf (plus some other formats, not really important to me). Sony 300 supports the same, plus still its proprietary .lrx (DRMed) format. Owning about 300 Sony books, that's a huge plus for me.
When hearing about Sony switching to .epub, I've been worried whether my old Sony books still would be accessible (as with the .prc/.epub discussion for Cybook Gen 3, having to go for 2 firmware versions).
Page turns and speed in general:
I have to do some more testing with various books (identical on both units). But so far, Opus seems to be a bit faster than Sony 300 in page turns.
Opening books seems about the same.
Initial bootup (building the library of ca. 300 .epub and .pdf) is relatively slow on both units, Sony 300 seems a bit faster.
UPDATE: Sony 300 for turning pages is slightly faster then Cybook Opus, but Opus still is fast enough (faster than Sony 505 or Irex iLiad for example).
Touch and feel:
Maybe it's because I already own Cybook Opus for a month now, but I slightly prefer the page turn buttons on Opus to Sony's solution.
I usually read in portrait mode and conveniently use Opus' buttons on the right with my right thumb.
For Sony, I have to change my holding position, when using the buttons on the bottom.
Build quality:
I won't open the Sony, so I can't really tell yet.
With Cybook Opus (I'll add a picture in a few hours), I'm a bit annoyed by the battery and its compartment. In BeBook for example I can use a standard NOKIA battery. In Cybook Opus, theoretically the battery is user-replaceable. But it's not a standard battery. The battery is soldered with 2 wires. Maybe because of the small size they had to rotate it by 90° and therefor needed to connect via those 2 wires. But to me it simply looks extremely fragile and 1 of the 2 wires already is a bit loose. I wish, I never would have seen that...
In general, Sony looks more "stylish", but I can't see the inside...
UPDATE: There's a difference between Sony 300 and Sony 505 or Sony 700: Sony 300 has a "plastic frame" (snapshot on page 12 of this thread). Meaning: In side view, it's entirely "surrounded" by a plastic frame. I guess, that's more scratch resistant than being entirely of aluminum. But I'll need some time getting used to it - 2 materials, not 100% of the same color.
User Interface:
Most of the thumbnails of .pdf on Opus are "distorted" (see the pictures in my Opus thread). Haven't seen such "blemishes" on the Sony so far. Thumbnails for .epub on Opus are fine.
Opus has folders. Not perfect, in my opinion, but at least it's got them.
No thumbnail view on Sony, just the sort criterias we already know from Sony 505: Date, author, book title.
Momentary result:
Sony 300 definitely looks way more appealing. In daily usage, both of them are very similar.
Owning a lot of Sony books already, I'd go for the 300 if I would have to choose. But both of them are fine units.
Update: For comparing sizes, I've added a pic Sony 505 vs. Sony 300. Not that much of a difference...