Teh CB fits better, so temp control is a lot better. you can also get the modelt hat works on a WSM as well, making it more versatile.
As for he motor, I am told that the battery powered one works just as well, so no cords is a bonus.
Supposedly the Weber one isn;t made for a Performer, because of the nuts that hold the lid bale.
They may have changed that.
You're not liley to find a used CB unit, but you do see the Webers on Clist every so often and a t a good price
If price isn't an issue, I would get the CB.
AJ