Y'all seen my dogs plenty of times by now, so I'll skip the pics. I think any Pitty mix can be an excellent watchdog.
A pure pitbull, well, as CDH stated, he might make some noise but he's more likely to lick anyone to death than anything else. The caveat to that is that he'll defend *you* like a tiger, but your property? Eh, not so much.
Then with a mix it depends on what you're mixed with. Our little one (55lbs) is a Pitn / Boxer, and we love her but she's dumber than a bag of hammers, eats bees recreationally, and is prone to barking at nothing at all.
Now Big Dog (90lbs) is a Pit / Rott Mix and lemme tell you, if you ever find one of these guys at a shelter or a rescue, take him home on the spot and love 'em forever. I've owned many, many dogs and I'll say, if I could clone Jim I'd make a million dollars in nothing flat.
He's got the family dog friendliness of the pit breed, with the perfect amount of the Rotties aloof intellect. He's alert to what's right and whats not, mellow around kids, and smart.... too smart sometimes. Plus he's got that pit bull heart, once it's in his head to do something, he's gonna do it or die trying.
I love that dog like family, he'll be twelve this summer, dunno what we're gonna do without him, I try not to think about it too much.
A note, both of these breeds are *very* protective of the women of the house, so probably not a good call for a single lady if you have "friends" over from time to time, they're likely to decide you need to be saved from your boyfriend and become aggressive.