Size, temperament, overall build. I would really recommend Shastid and Roaches book on the Bullmastiff if you are leaning anywhere near them.

To me there are a lot of differences. And drool will vary with individuals and tightness of the flews. Just like with Saints.

I have seen very bully mastiffs and mastiffs with long horsefaces. It really depends on the lines. But generally they are bigger than BMFs and some lines tend to be guardier or edgier. You really would want to know the breeder and what they want in a dog and are breeding for.

Also, my BMFs breeder had dogs that lived 12 and 14 yrs. I have a friend with an 11yr old Mastiff. At the same time I just lost a Border Collie at 11. Sad to lose the big guys but not something I will soley base my picks on.

IMO the first pict is a mix at best and possibly not even mastiff at all. I have seen Plott hound X Labs that looked like that dark brindle. The second fawn is a decent representative of a Bullmastiff.

Personally a Bullmastiff is on my list of DEFINITELY OWN AGAIN after our two passed I KNOW I will have another...when the time is right.
Why I don't want a mastiff....too big, Generally too slow to respond and too big. Likewise too many joint troubles and too shortlived.

Mixes...great if you get the right combo but genetics are a fickle thing. Look at the diversity in purebreds...then add a whole nother breed. If you pick a pup will it have the mastiff temperament or the 'other' breeds? Will it get big? Drool?
I have a client with a Saint they loved so got a Newfie mix. They got the mix thinking how much they love their Saint....yeah, but the mix pup even though it was a big pup didn't grow and turned into a lovely thick and long coated hyper brainless lab....not what they wanted at all.

I like purebreds.