Well it depends on what you want in a phone. The D600 has 850, which is a definite plus over the k750i. The k750i's camera is better, the UI is better, it's cheaper, has more options for memory. The D600 has a beautiful screen, 850 band, and cool formfactor.
Here is a cam comparison done by GSMArena.
http://www.gsmarena.com/review.php3?idReview=51&idPage=4