Advantix is Advantage plus tick protection. It's more dangerous than Advantage alone (which is only rated for fleas, though I and others have found ticks not to be a problem while using it, even though it doesn't say it's for ticks).
Advantage is safe for puppies and kittens and for nursing mothers. I believe it is the safest spot on flea product out there. However you have to be careful with Advantix (which I have also used) in that it is lethal to cats. If a cat licks a dog or has physical contact with a dog and then licks itself while the chemical is still wet, it can kill him pretty quickly. I do have cats. So the two times I treated with Advantix I got paranoid and locked the cats in the basement and the dogs in their crates over night. By morning the dogs were completely dry.
There's a permethrin type chemical added to the Advantage to make Advantix and it's that chemical that is toxic to cats.
Advantage = very safe, but officially only for fleas, not ticks
Advantix = safe for dogs, lethal for cats, works on both fleas and ticks