I've shopped from both and love them!
Whittles carries high quality quilt shop fabric such as Moda, RJR, etc.. They have wonderful prices and a huge inventory.
Connecting Threads carries their own line of fabric. While not LQS quality it is far and above anything you'll find at Walmart, Joann's or Hancocks. I also love their thread and it is price really low.
So, that's my two cents worth! :thumbup: