Depends, no one breed is 'better' than another. AmStaffs and Rottweilers are both good breeds in their own way.
AmStaffs are friendly, outgoing dogs, but can be quite dog aggressive. Most well-bred AmStaffs are great with kids, but are not 'guardian breeds'. Meaning you shouldn't trust an Amstaff (Or any bulldog) or guard your home if the need ever arose.
Rottweilers are good dogs, but are a 'guardian breed'. Meaning most Rottweilers are aloof and wary with strangers. However, most rottis will guard your home if needed. Rotti's can be dog aggressive, but were never historically a 'fighting' dog, so it isn't as a common trait in Rotti's as Amstaffs.
It really depends on what you are looking for. I dont know to much about amstaffs but i do have alot of experience with rotties. I would do some research and see what breed would fit into your lifestyle better.
I have a rottie/husky mix, he is very good with other dogs ( I have 3 others) and he loves the cat, but strangers...NOT SO GOOD.. I had him before I joined this forum and did not know about socializing because I've never had a problem with any of my dogs with other people. When anyone comes over he is not unaquainted with, he must be put into the Laini. He is 5 years old now and very protective of his home. Luckley I do not have too many strangers come to the house. He's great with people he knows. So if you get a rottie, get him used to other people from the get go ( animals too)