Great comparison, HarryT. I'd add that the iLiad has an input method in its pen and digitizer, which can be used to annotate while reading-- not everyone needs this feature, but it is rather a huge difference between the two. Other differences include the wireless networking on the iLiad and the variety of storage types supported (CF, SD, USB memory). Again, these features aren't of interest to everyone, but they are worth noting, I would think.
You might want to try FBReader, ported to the iLiad by our own AdamB.

I use it for reading all non-PDF documents. While I miss the annotation capabilities of PDF when using FBReader, I like the library function and the general usability of the reader. I'd be interested in a comparison between FBReader, Mobipocket, and the software on the Sony.