It's very, very sad if displaying manga and comics is one of your primary worries on an expensive device like th Sony or even more expensive device like the Iliad.
(Quietly closes directory with over 250 coverted issues of both....
To be honest, after looking at all three ways to view them electronically I would say tablet>Iliad>Sony (but to be honest, only had a couple days to look at the Iliad).
Tablet of course will have the best image and resolution as well as the most options for viewing.
The Iliad is 16 grey scale which is more than good for most color comics to still look good and more than enough for most manga and B&W graphics. The larger resolution size (1024 x 768) means less detail is lost during converting and display when compared to the Sony (800 x 600) meaning more fine details and some fonts are visible.
The Sony is a 4 grey scale which is still surprisingly good for some color and most B&W graphics. The smaller resolution and size does make a noticeable difference though on the final viewing for PDFs (mostly with some finer fonts being hard to read).
As you can find out reading from the Wiki (button at the top of the page) and through out the various threads, there are several good utilities for cleaning up images and PDFs that work very well in making files look much better for display. They work well enough that MOST of the inherent problems that come with a smaller, lower res screen can be mitigated.
I have a Sony ereader, seen the Iliad (love that display and ergonomics), tested the same mangas on both. Final opinion: with proper formating and conversion, the Sony is a good device for manga with font display being the biggest problem due to some fonts used while the Iliad is a better display for this purpose with all fonts being readable and more detail showing.
Having said that, is the Iliad 3x better (being the cost of an Iliad to the Sony)? Not for me. I like the Iliad. I really, really do. Unfortunately being Stateside I can't find a way for me to justify the higher price plus shipping as well as the overseas service required if something goes wrong. Not just for entertainment. This would probably be a different story if there was a U.S. vendor but at this time there isn't.