I actually like the Kong better, physically, but the CT RollNLock is rated better as a climbing device. Therefore, I chose the CT RollNLock over the Kong. I have used both, and didn't have an issue using the Kong (I used one for a while). The CT was kinda like upgrading my insurance policy.
I would say that the Kong is lighter, slimmer, slightly quieter (without minor tweaks), and has a stronger return against the rope. The CT is slightly heavier, a bit wider, has more uses, needs a pull cord added for ease of use (also quiets the pulley, if it's done correctly), and will lock open if you choose. I haven't noticed enough of a difference, when actually using them for climbing, to say that one "works" better than another, though.
I use them, backed up by a stopper knot and/or prussic (or tied into my carabiner), for both my lineman's and my tether.