Kona has a firmer hand, Bella is softer - to me this makes Bella a little harder to work with sometimes (esp. on bias cuts), but in most situations there's no real difference (and you can always use some starch to firm it up).
Both sew up beautifully and seem to be of similar durability and once they're in a quilt I can't tell them apart - if you can get a good deal on one over the other, I'd probably go with the good deal. I personally prefer Kona but that's just my preference.