You should also look at your location. If you're in a very hot part of the US, I wouldn't go for the bigger or more brachycephalic breeds like Bullmastiff or EM. I think many of those dogs are not keen on a lot of hiking etc. either. A BB might suit. A Tosa might be ok, but you'd have to worry about meeting up with strange dogs. Whatever you get, it should be the opposite sex to your Lab. Most molossers are dominant dogs, they tend to get along much better with opposite sex. And no heavy packs for dogs until they're past a year old ;)