Basically each person has other priorites when it comes to E-Readers.
I started with the PRS-505 and it was/is great. But i always wanted a bigger screen and the dictionary. PRS-900 also adds pan&zoom for PDFs which are important too for me.
Now that i own a PRS-900 i love it. Although i can't use 3G (non-US) i'm fine with that; not important for me. Due to Apple and others the PRS-900 is cheaper than ever.
Personally i recommend the PRS-900.
Go get one, and you'll love it.
If you need games, internet, color....that's another story and device.