What should I do? Is an American Staffordshire terrier or pit bull right for me?

I am looking for a new puppy. I currently have a Boston terrier and an elderly boxer. Both are trained very well. I am good with training animals. The Boston can do multiple tricks includimg army crawl and ring a call bell. We have a green cheek conure that I have trained as well. I have trained the dogs to ignore the bird and they do, they know I mean business about that rule and they listen. It is for my birds saftey.

Im considering adding a third dog to the mix. I feel like dogs learn from dogs. I need my elderly dog dog to help me teach a new puppy manners.

I am consider a. American Staffordshire terrier or APBT. Im reading bad bad things about them. I like a challenge but i dont want to bite off more than I can chew.

Which breed is easier to train? Easier to socialize with other dogs? Long term goal is rally and obedience competition. I need a biking and running partner. My boxer can’t anymore and my boston has hip dysplasia and has been through surgery and she can’t keep up with me.