I'm pretty newish here too but I did read on the kobo site that you can buy books to transfer over to the Sony ereaders too
http://www.kobobooks.com/ade
They only have the older models listed but I'll assume (and probably make as A$$ of myself) that they'll support the PRS350 & PRS650 too (hopefully someone can confirm that)
that being said I pre-ordered the Kobo2 and I'll probably either buy one of the two new Sony models tomorrow. I'm still up in the air now with the Kindle3 as I'm worried about geo-restrictions though but it does seem like amazon.com does have a great selection of ebooks.
Welcome too
Fiona