Hi Katania.
I'm in the US, but I will opine about formats.
The Sony Reader uses its own format, LRF/BBeB, and I find it very natural and readable. I appreciate it more and more as time goes on. The reader also supports RTF and PDF. While I have not used many RTF books, PDF is... let me see how to put this... an unmitigated disaster. I cannot stand reading PDFs on the Reader -- even the Reader's Manual!
If you do decide on the Reader, I consider converson to Sony's LRF/BBeB format imperative. And since the Sony Store and ManyBooks.net are the only places that I've found supporting this format without conversion, you're somewhat limited.
I have no experience with the Iliad. I'd ask how PDFs look on that -- though I don't imagine them looking better, since it's the same e-Ink all around (maybe they have better software?).
Maybe somebody else can give their feelings on RTF as well.
But for me, it's become Sony's format or nothing... so much so
I wrote a program just for this purpose! 
-Pie