If you are referring to rim inches, then the 32' is much better as it has more stability than the 40'. The 40' raises the truck which can cause problems when loaded. Check with a mechanic for more advice.
Depends on what he's using the truck for as well. if it's every day use go for the 32s but if it's for digging in the middle go for 36s. Me personally I don't recommend 40 I less the truck has the transmission and gears enough to turn her over and support the bigger wheels the bigger you go the more turning / pulling power you need. So if she's just a standard factory truck with no modification stick to the 32s or 36s to be sure it can turn them over without tearing the rear from under her.