If PDF is more important than books for you, then you should definitely go with Sony (see PDF section in
Kindle vs. Sony comparison). Sony was better at PDF even in PRS-600.
As far as K3 PDF features making it into KDX are concerned, my guess is that Amazon is currently working on software update that would allow to lend books. Once this software update is released, all devices starting from K2 will get 3.* software and hence identical PDF features.