I bought both from that site. The Sundae Swirl is softer than the Bella Color. I started a sweater in the Bella, and it's ok, but not super soft. It's not harsh or anything, just more plain-cotton feeling, I guess.
I have only knit a tiny swatch with the Sundae Swirl, and it's smoother. I think I'd go with that for a baby blanket, especially.