Just to add to your certainty: If you want native ePub without having to convert to mobi through Calibre, then your only choice is a Sony reader, as Kindles only read pdfs and mobi, Amazon's proprietary format.
Happened to test pdfs on my PRS-350 this morning and the options for them are amazing -- superior to those of the K3 in most respects and as good or slightly better in one. The x50 series has both slider and gradated options for zooming in and out just as they do for brightness. Like the K3, it doesn't give you a slider for centering pdf text, which means you might have to muddle through larger pages if, for example, you're trying to read a rare photocopied book turned into a pdf and have no other choice of format.
Take, for example, this two-volume edition,
The Poetical Works of Thomas Lovell Beddoes, which dates from 1890 and of which a total of 750 copies were ever printed. Unless you've got one of those few copies, the one version without horrible typos that you can actually read is available for free
here.
When looking at a file as problematic as that, I do find the size of the reader matters. But in the case of pdfs that are fully manipulable and contain embedded fonts, any size reader seems to work fine.
One thing you could do would be to bring an SD with you to the Sony Store, insert it into demos of both models and try looking at a familiar pdf file on each.