One reason for staying with a X50 reader (at least that's MY reason) is PRS+
In future versions (yes, it's still under development) improvements like this are to be awaited:
And hopefully user-dic's...
I use the new layout-dialogue in my Zork-edition so there is little reason to change my reader.
New screen resolution of Paperwhite in comparison with Sony:
6" reader from Sony: 166ppi
5" reader from Sony: 200ppi
6" reader Paperwhite: 213ppi
213ppi is noticably better than 166ppi, the typical resolution of todays 6" readers. But in hinsight of the PRS350 I see no huge improvement. So yes: 'good old Sony'...