I purchased the Clam XT 200 Thermal Pro this year and have used it 3-4 times. I love it and it comes with all the extras that the Otter does not ie: Skids for the sleigh, lights, battery holder, etc...
I looked extensively at both Otter and Clam before making the choice to go with the Clam. Overall the material was of better quality and had more insulation, the tub seemed tougher, and the pole system seemed sturdier on the Clam.
I don't think you would regret either one.