Whichever one is on sale... LOL
I prefer Kona but I'll use either and have used both in the same quilt many times. Bella feels softer, more drapey, which is actually kind of a pain while quilting. Kona is a bit stiffer, making it easier to piece. Once they're sewn into a quilt, I can't really tell them apart.
I like Connecting Threads solids too - the colors are a touch "muddy" compared to Kona, IMO, but they're still good quality for a good price and if the color works, it works. You can order a charm pack from Connecting Threads to see how their solids look in person - they don't charge extra for cutting the fabrics so it's a pretty good deal. I buy my solid white almost exclusively from CT now - I buy 8 yards at a time and wrap it onto a bolt. It's a little thinner than Kona but thicker than the "bargain" solids at someplace like Joann's.