Being a solids junkie, I've used both Kona solids and Bella solids...together in the same quilt even. They do play well together, despite their subtle differences. It's almost as if Bella is the feminine version of Kona.
Both are 60x60 broadcloths, but Bella is not as 'harsh' as Kona and she has both a softer hand and a smoother, almost glowing finish. If I were mixing with batiks, I'd use Bella, hands down.
That said, Kona has gotten thinner over the years (like what fabric hasn't, right?) and now works much better than it used to with regular quilting cottons, especially after it's washed since that tightens up the weave quite noticeably.
I don't prefer one over the other, but rather choose based on the project, the price, the color, and the availability. I always buy solids in person when looking for an exact color coordination, which is usually the case, and have also used the RJR solids as a result. They are comparable to the Bella.