Floeee,
If you go to the irex website you can see the differences between the models:
http://www.irextechnologies.com/products.
The main differece between iLiad and iliad book edition is the latter doesn't have wifi connectability. (whether it is of any use, I suggest a look a the iliad forum and make an assesment on how you would use the device. The software is no longer supported by irex.
The DR is bigger and faster (no wifi) and more suited to a work environment.
The speed of response, between irex cevices and Sony, I cannot comment on other than in general terms.
I have a Sony 505 and for reading epubs/mobi books on the DR1000 and Sony 505, they are much of a muchness in general use. No great difference in practical terms. However for pdfs, the Sony is hopeless for A4 or larger. I cannot read them at all comfortably and the rendering isn't quick. The Sony also takes a long time to move to Landscape which you would have to do to read. (Although that is rather dependanty on your eyesite and the natural size of pdf). The DR too can take a while to open a pdf but is generally reasonably quick to move between pages and I don't have to change to landscape. I can zoom and pan on the DR which I cannot do on the Sony 505.
However, I understand the 600, having a touchscreen can handle pdfs somewhat better, but I'd suggest a carefull read of that forum.
Nevertheless, If you wish tor read "Letter" or "A4" docs, and can afford it, (it is on the pricey side) I'd still buy the DR. That's not to say things may change as other developers come around.
If you just wish to read a few large pdfs and a lot of books, I'd consider the iliad. However, for book reading the iliad is limited to mobi books whereas the DR will (sometime soon) be able to read epub and epub is the way Europe appears to be moving.