Hi Nigel
I have just bought the Iliad (previously owned a PRS500). The 2 devices are fundamentally different. PRS500/505 is extremely good for the casual user for reading fiction because of its long battery life and smaller size.
Iliad is a work tool. The important point is that viewing of pdf files is not a problem on it. I believe that the slightly better display on PRS505 is not really going to enhance the reading experience a lot. After a while, our eyes will get used to it anyway.
Also, the battery life is really not so bad. I used mine for reading in trains while commuting to work(about 40mins a day, page turning+touch pad) and turn it off after that. Charged it 2 days ago and now it is still at almost 80%.
In fact, Adams has just released the beta version of minimo browser. This will really bring Iliad to an even higher level because now we can have web surfing similar to that of using a pc.
Best regards
Quek