I agree that "more memory is better," although I don't see much of an advantage to an SD slot. I suspect it costs as much to put in and maintain the slot as it would to just add the RAM, plus it slightly increases the complexity of a class of device that IMO should be as simple as possible. I expect (but may be wrong) that adding either more RAM or a media slot will also result in higher power consumption.
Even with PDF support though, the K2's screen is too small (for me, at least) for extensive PDF usage. To me, that's a much more serious imposition on PDF usage than storage capacity. If you're using PDF's all day long, I'd think you will need a larger-screen device anyway.