Have you thought about a Greater Swiss Mountain Dog? We are bringing one home next year (fiance's dog) and really enjoy them. We have yet to meet one with extreme dog aggression, though I have been told some can be a bit more dominant then others (but this is true for any dog really). They are laidback in the home -- every breeder went out of their way to assure us the dog would be relaxed in the house with proper training, and they are not very highstrung or hyper to begin with. This was important to us because of the baby that will be here, and after owning hyper dogs in the past, it's a trait we didn't really want to deal with again. Also, they are great hiking companions, and can carry packs and such :) From the breed website: The Greater Swiss Mountain Dog is an incredible watchdog. They tend to notice everything in their surroundings and are very quick to sound alarm. Faced with a threat, they will stand their ground and put on a show that will intimidate those unfamiliar with the dog. However, Swissys are not guard dogs. A Greater Swiss Mountain Dog should be very bite-inhibited. He should be reluctant to bite, doing so only under the direst of circumstances. The Greater Swiss Mountain Dog is a very dependant dog and one that needs a good leader to follow... Given a qualified leader figure, the Swissy has a fairly submissive nature and is a willing worker happy to follow through with the tasks at hand. Walking, hiking, back packing, swimming and carting are all good opportunities for exercise to keep a Swissy in good physical condition. I think that they would be a good fit for you and your other dog. I wasn't sure I really wanted one at first, but after meeting them at shows and in breeders' homes, I am really excited for ours and think we made a really good choice. And yes, from what other have said also, a lot depends on your area. Lots of the larger mastiff-type breeds don't fare particularly well in hot weather. We are in MI so it doesn't get too bad here.