I dont have personal experience with those particluar breeds but have had friends & neighbors who do. I do know you have got yourself a BIG puppy who will continue to grow for a couple of good years. So feed him a food that targets LARGE breed pups and helps to keep the rate of growth in check. Give him lots of love and play to keep him engaged and use up that puppy enegry and lots of those hard rawhide chewy sticks (I only use "American made" beefhide as I do not want to give my dog something from China or Brazil where they do not have the regs we do here. Find those at Meijers...sometimes Menards. It really is amazing how much of our animal products are made in those countries if we would just turn the bag over and see.) to help him thru the 2 sets of teeth they deal with the first 6 months of life. No wonder they chew everything!
He might be a drooler with the St bernard, so be prepared for that it is part of the breed. Just keep paper towels on hand. And if you will not want him on the furniture when he is an adult, do not let him on the furniture now.
Both breeds are extremely loyal, eager and willing to please. Be sure to socialize your puppy very well at a young age with people and other animals. Both are highly intelligent and easy to train, however training should begin early, while the dog is still a manageable size, and you have him early so start now. Teach this dog not to jump on people starting at puppyhood. What might be cute as a baby is NOT cute as a 100+ pound adult. Bear in mind that an unruly dog of this size presents a problem for even a strong adult if it is to be exercised in public areas on a leash, so take control right from the start, teaching the dog to heel. Both are excellent watchdogs, but be sure and teach that YOU are the master and he must listen. Patience & Love are key and little treats help your dog focus. They are in it for the treats and the attention