I've used both...together and separately...often.
They are both 60x60 broadcloth. I have found shrinkage and fraying to be comparable now that Kona is thinner. I can still find Kona at a lower price than Bella.
Bella has a softer hand, a hint of glow in the finish, and is what I would call 'refined'.
Kona has a more coarse hand, a matte finish, and I would call it 'sturdy'.
Bella has not yet grown to the vast color assortment of Kona, but they probably will in due time.