If you look at the feature list, the Iriver will bring more to the table. The 300 from Sony doesn't have touch (neither fingers nor stylus), no memory expansion (which wouldn't be a problem with novels primarily as these are small files) and it is smaller (5" instead of 6").
On the other side, the 300 brings better portability and as far as I hear, its display is like the 505 which I loved. In terms of later development or hacks, I would probably tend to the Sony. But I would also love to see an Iriver in real life. It's the same with sports cars. I would love to test drive one, but I don't really need or have the use for one.
So if you compare functionality, I think you have to compare the Iriver with the 600. In this case, I think the 600 is the more finished product.
If you look at it from a "I-only-want-to-read-novels"-point of view, you can compare the iriver with the 300, too.
Actually, if the Iriver has more features than the 300, a similar price and you still think you only want to read books, I would probably go with the Iriver. You never know if you have some use for the additional features later. Usually I like the idea of getting more features for the same money, if quality is not an issue.
I haven't read all Iriver threads here, but what I read so far, for reading only it seems ok. And maybe there'll be some more FW updates later.
Creative (one of my older mp3-players) updated the FW every 3 months in the beginning to get all the bugs out. After some time the FW updates got less, but by then, the mp3-player was a solid and stable as any other player.
Sorry if I don't get more specific in recommending a device. But I really think it strongly depends on your personal preferences and usage.
Ok, long post. I hope I could help making your decision a little easier anyways.
Take care
