Phillo, for most it is easiest to just get the dog neutered when the vet says to. The big arguments about not s/n at 6 months are mainly for people who do dog sports. Agility, flyball, herding etc can all take a toll on the dog's bones and joints, so people want to make sure that the dog grows and matures to his/her full potential before taking those hormones out of play.
Not sure of the exact specifics of rates of testicular cnacer, but I have known more than one person who had to get their dog neutered due to testicular cancer. I don't think the rates are very high, but it is a risk in all unneutered males.
As long as both of your dogs testicles are descended you can keep him intact as long as you'd like. A dog with an undescended testicle is at a higher risk for cancer and it's best to go ahead and get that taken care of. The thing with owning an intact male is you have to realize how quick a breeding can take place. You have to be completely sure that he cannot ever get out of the fence, and if you walk him offlead anywhere you have to be extra vigilant to keep an eye out for stray dogs.
If you have the means to responsibly keep him intact and you'd like to wait until his growth plates close before neutering, the optimum time for that is 18-30 months.
Just weight your options and do what you feel is right. I will tell you, I have only ever owned one unneutered male. He was neutered at about 20 months. All our other males were neutered ASAP because there was no research to say otherwise. We have yet to own a dog with hip dysplasia or any joint issues aside from the aches that come with old age.