For me it's the Sony, even though I hate supporting their philosophy of trying to lock users into their software and store. However, the community support for the Sony is awesome, starting with Calibre. Just root the reader right out of the box and be able to enjoy Android apps, with MY choice of reader software, MY choice of pdf support and tons of other apps straight out of the Android market. Can't beat that. But that's me. You may have different needs.
Wiki - PRS-T1 rooting and tweaks
Wiki - PRS-T1 working apps
Come to the dark side...
edit: oh and one thing I also absolutely hate (but I think it's most of the mainstream readers) is that the battery is not (in theory/easily) user replaceable. I liked that in my Hanlin V3 I could just buy a new standard Nokia battery and be done with it.