
Hi guest,
I'm not quite sure why you want a protective pet, but I'll answer your questions as honest as I can.
I have owned, bred, shown & done obedience with Bullmastiffs for many years.
I have owned,shown & done obedience with a rottweiler (but never bred rotties)
Slobber, yes bullmastiffs slobber a lot, it's something you have to learn to live with & whenever you go out, you'll learn to always take a slobber rag with you.
I have children & cats & never had a problem, but as with any dog providing the dog is brought up with them from a young age, there should be no problem. Remember these are large dogs & care should always be taken with dogs & children.
Off the lead in the park, I'd say it really depends on the individual dog, but if the dog has been well socialised & is obedient, with excellent recall, I see no reason not to let the dog off lead for a free run.
Life expectency, Niether are long lived breeds, I'd say the life expectancy of the bullmastiff is probably not quite as long as a rottweiler - although each individual dog is different.
In my experience, most of the Bullmastiffs I've had will happily guard the house & bark if there's anyone around, but that's it. Rotti's (again just the ones I've known) are more protective, my big lad would have risked his life for me, he was well socialised & I didn't want him to be protective, but that's the way he was & he'd guard the home / car & me with his life!
Intelligence, oh boy, I don't want to offend anyone in saying this but my honest opinion is that Bullmastiffs are lovely but not intelligent at all. But rottweilers I would say are extremely intelligent, as I said earlier I've done obedience with both breeds & in my opinion, I'd rather work a rottweiler anyday, bully's are happier curled up by the fire. A rottweiler is happier with something to occupy his mind. I've also seen rottweilers working as police dogs & spent training days with them, they work for the love of it & enjoy being stimulated.
WOW - hope I haven't offended anyone saying bully's are not intelligent, perhaps it's just mine that had no brains.
Hope that helps, Both breeds are beautiful but please go to a reputable breeder. I think temperamentally the Bullmastiff is more sound. I would want to ensure the temperament of the sire & dam were very good before buying a pup.