Well, I think you've gotten a pretty thorough answer, Bierius, I don't have much to add, except an observation about the approaches of the two companies.
Since both units use Linux, homebrew 3rd party apps are pretty much inevitable -- just look how fast these maniacs (meant in the nicest way
) started hacking their 650 Euro devices!
So I think a point to consider is the approach the two companies are taking to those apps.
iRex specifically designed their device primarily to run 3rd party apps, but they seem to want to keep some control over
which apps those might be. It's still unclear what they'll do with that (since that hasn't started yet), but they've said something to the effect that they want to deliver 3rd party apps via their IDS. I don't think anyone knows what they'll try to do with apps that are passed around beyond their reach. My guess is probably shrug and charge for de-bricking service as needed.
Sony, on the other hand, hasn't really said diddly about third party apps. There have been some wisps of rumour about quiet talks, but not much there. However, they also haven't (as far as we can tell) put an equivalent of the IDS in place to feed their readers apps -- we do know that they can send firmware updates out to their units via (wired) 'net connection.
Okay, here's where I get
really speculative, so salt to taste.
I think that you're more likely to see Mobipocket or eReader (etc.) on the iLiad through official channels than on the Sony, however, I also think that you might be slightly more likely to see "unofficial" readers for those formats on the Sony.
That's kind of a "well, duh," because iRex will officially allow new apps on their reader and it appears Sony isn't thinking along those lines, at the moment, anyway. But they're both talking some sort of SDK release, so it would appear that Sony doesn't plan to just lock us all out of their device. (shrug)
All of that being said, I think you're likely correct that the iLiad's screen size is going to make it the better choice for manuals and journals of fixed A5-ish size pages.