It has been researched that two males, whether neutered or not, are more likely to fight than females. Two males that are intact will fight the most.
Two females are less likely to fight, BUT when females do fight it is generally a more serious fight and it's more likely a serious injury will happen.
If I were you, I would pick the puppy by personality. All litters have a mix of different personalities. Don't pick the pack leader out of the puppies. A puppy somewhere inbetween in terms of dominance would be best. If you search online you should be able to find puppy aptitude tests. Also talk to the owner of the litter because they will know the puppies best. Also, some mastiffs are aggressive and dominant, so be careful when getting one...research different lines first.