Pitbulls seem to be more prone to aggression. I know this is profiling and it is sad for the dogs that they were bred this way. I never allowed my child near a pitbull or mix. I have come across many that seemed like sweet, affectionate dogs, but they have a trigger that makes them violent and it is not always easy to tell what that trigger is exactly.
Any dog can attack/bite. Small breeds do less harm. Larger breeds are stronger and able to do more harm. Then of course pit bulls - real or perceived - tend to make the news. But multiples breeds and mixes of breeds are called pit bull type dogs - sometimes on appearance alone. But it is impossible - even for dog experts - to accurately tell the breed by appearance (this, according to the people who should know) and many experts oppose breed specific laws because they are ineffective. Montreal's law includes in its definition certain physical characteristics that are found in numerous breeds aside from true "pit bulls".
Every single dog on this planet has a trigger that makes them violent - even the most good-natured dog. If a dog feels threatened or if it feels someone else is threatened then it is more likely to attack.
It's always wise to be vigilant with any dog - especially larger breeds - when there are people around that might be unable to defend themselves like the elderly and especially young children since their behavior can sometimes be perceived by the dog to be threatening when it is not.
I have 2 dogs - one a lab mix, the other some sort of mix that would definitely be perceived as a pit bull type dog based on appearance (no DNA test at this time). She's been a good dog and is very good in public. I am very cognizant that since they each weigh about 80 lbs - give or take - either could do a lot of harm if they ever decided to do so. They are always on a leash in public and I stay aware of our surroundings.
Most people are going to believe what they believe no matter what the experts say, but quite a number of them are against breed-specific legislation. It is what it is, and you certainly have to do what you feel is right and will keep you and yours safe.
A slight correction to my comment above about DNA: that was another attack in which the owner ordered the dog to attack his wife (or ex-wife). The attack credited with inspiring this law was a dog that was not properly restrained and somehow got into a woman's back yard, where it attacked and killed her. That dog's DNA test is pending.
Cesar's take
https://www.cesarsway.com/about-dogs/pit-bulls/how-did-pit-bulls-get-a-bad-rap
This article explains the difficulty with labeling a particular dog a "pit bull type".
http://www.huffingtonpost.com/steffen-baldwin/the-lies-damn-lies-and-st_b_8112394.html
Pit bull facts
http://www.caninejournal.com/pit-bull-facts
ASPCA statement on these dogs
http://www.dogsbite.org/pdf/aspca-pitbulls-nursemaid-dog-july-2014.pdf
