I can't believe I'm being this much of a stickler, but the Greater Swiss Mountain Dog is a different breed than a Bernese.
Greater Swiss Mountain Dog - Wikipedia, the free encyclopedia
OP, I'm just curious, what is it that is drawing you guys to the two breeds? You're looking at some pretty different temperaments, there. You're getting a lot of info on GSDs, so I'll just throw in a little about the GSMD. They're a livestock guardian breed, which means they were bred to have a natural protective instinct. I don't think they're as bad as some of the more hard-core working livestock dogs out there, but you have to be VERY aware of what that protectiveness entails. Where a GSD is protective but bred to focus very much on a handler, livestock dogs are bred to literally live out with their flock. They ARE bred to be very gentle with whoever they are raised with, but not bred to focus on a handler. They are meant to make their own decisions on when to be protective because a predator can attack in the middle of the night and they need to follow instinct without waiting for commands.
What this means for pet owners is that the dog needs RIDICULOUS amounts of socialization. It takes a little more effort for training because the dogs aren't as naturally in tune with people. They will be super wonderful and loving with the family they're normally around, but can really easily develop guarding problems over "their" property. This can be as harmless as barking at people who walk past the house, or as dangerous as getting protective over your children while they're playing with friends. You really need to be diligent about not only taking the dog out of your property regularly for socialization and training, you want to actively work on training this type of dog to accept strange people in your house.
Like I said, I think this breed doesn't have as many issues as other livestock dogs, but you still need to be aware of the potential. They might look sweet like a lab or a golden, but they are definitely different dogs.
With a 4 month old baby at home (which is an awful lot of work!) you really want to objectively look at your situation, the time you want and can realistically invest into training, and the traits you're looking for in a dog. With your current dog, you might have a stubborn trainer on your hands (beagles and pugs are not really known for their smarts! haha), but neither dog is going to be naturally protective or aggressive. Now you're looking at two breeds that both have the potential to go pretty poorly unless you're really diligent with training. It's a really great idea to make a list of the things that are MOST important to you in another dog, then start picking out breeds that fit in that list.
Not saying either breed necessarily ISN'T a good fit for you, either, since I don't know about your history with dogs or how much you want to invest in training! Just that it's good to think of all these things.
Also, don't forget that adopting an adult can be really great! You'll already have an idea about the temperament of the dog, so it's possible to find a dog that has the potential to be more difficult, but that will fit in well with your family. Good luck on your search!