Bella has a softer hand and a smoother surface finish than Kona. If we were talking paint, I'd say Bella is satin and Kona is matte. Both are 60x60 broadcloth, but I suspect Bella's threads are finer than Kona's...no proof of that suspicion at all however.
Kona softens and the weave tightens up with washing, Bella's hand stays much the same as off the bolt. They are both excellent fabrics.