I'd choose the K-r, and I'm not just saying it because I have one
My best friend chose the d3100, and she hasn't a clue of how to use it, LOL. I thought maybe I could help her out with it, but couldn't; it's too darn complex in terms of user interface.
Of course, it has a guide mode and bla bla bla which will ask you whether you want a 'blurry background' but that's not very practical once you learn to use it.
plus, the shutter of My K-r sounds very film-like, which I love. the d3100's sounds electronic.:/