We've been using wood pellets which our new rescue cat used at CPL and were planning to change to Worlds Best eventually. However, we don't have any problems with odour at the moment and using the pellets has been ok.
This may be because I'm home and can clear her poo quickly but wonder if the clumping would be better for odour absorption if her poo was left unattended for longer periods?
Also, I wonder if the clumping material sticks to their paws and possibly transfers unnhygenic matter around the house moreso than the pellets?