My wife and I were thinking about getting a male Bullmastiff, as well. However, we ended up getting a male English Mastiff instead. Both are large, strong and powerful dogs, which is why they should be handled by responsible and preferably experienced owners. Based on our individual living situation, we felt that an EM was more "manageable" and "suitable". As you know, quite a lot of the behavioral traits depend on genetics, upbringing, and training, but I just didn't think we could meet the needs of an energetic Bullmastiff at this point. Again, this is all very subjective, and I cannot proof it providing any objective research, but I think Bullmastiff might be more reactive to both strangers and other animals/dogs. I don't know enough about the breed make a statement on compatibility with your dogs. Overall, I would utilize the forum, as well as other credible sources to read as much as you can. I have been extensively studying treats on the forum before getting my EM. I've read anything from training, to health, and behavior. Even though quite a lot of the information might never apply to your own dog, it can be helpful to informed about any possibilities. I have learned that many subjects (e.g. food) are highly controversial, and that available information can often be misleading or even contradictory.
Also, have you dealt with Bullmastiffs at some point other than your neighbors? I would also recommend to spend some time with them to learn more "first hand".
There are highly experience BM owners on the forum, so I hope you will find some more answers to your questions. Generally, I would always ask myself the "lifestyle" questions. In other words, can I meet all the needs of the dog, and can I take full responsibility for owning, training, and socializing such a powerful dog.