I have tried several different brands of pine shavings; I finally figured out that the smaller the flake, the better the odor control. I use Guardian Swift Pick, (actually horse bedding, get it at Tri State Vet and Pet Supply in Evansville, Guardian's website has a dealer locator) it's not much bigger than sani chips, but a lot thinner. About $5 for a large bale that lasts through 3 complete 6-tub rat rack changes. I think it has to do with the available surface area that absorbs liquids and odor better.
Just my 2 cents