Well the biggest con is definitely the relative brevity of their lives, everything else negative just pales in comparison
Of course there's hair, poo (including the inevitable accidents inside, sometimes when they're not well) and other bodily fluids, general muck (mud, nose smears, slobber/licked soft furnishings, unspeakable things they've rolled in on walks, etc.), endless training (not just for puppies, it's a necessary constant throughout their lives), inevitable training failures and the associated humiliation, vet trips and illnesses (which may involve having to do or deal with some pretty gross things sometimes), "grass/hair bum", grooming (how much is very dependent on breed), the smells (whether it's doggy smell, again it's breed dependent, dog farts or a faint whiff of sheep poo because you missed a spot when you bathed them after a walk) the sheer relentlessness of puppies (house training, chewing, biting, etc.) then just as you're getting over that they turn into teenage hooligans, the dog(s) always having to be sorted first before plans can be made.
It can't be all bad though, I've got four of the damn things
Seriously though, if you're prepared for the hard work then the positives vastly outweigh the negatives. You will never be as adored as you will be by your dog. A good dog walk is a fantastic cure for a great many ills, as is a lovely doggy cuddle. The satisfaction of having a well behaved, well trained dog (even if it feels like it only happens occasionally!!) is amazing. Doing activities and learning together is so much fun, whether you do structured classes or just try things out at home. Just the simple companionship is absolutely wonderful and well worth all those cons!!
If you're going down the puppy route then I'd definitely recommend meeting and chatting to owners/breeders of your short listed breeds, as well as meeting their dogs. Dog shows can be great for this and there are also events like Discover Dogs (October in London) and also a Discover Dogs area at Crufts, though you've just missed it this year.
Definitely don't write off rescues completely though. Quite often smaller, independent rescues are much more flexible with their rehoming policies and also rely on fosterers rather than kennels so have a much better idea of how a dog fits into a family environment. There are also breed specific rescues, often run by people heavily involved with the breed club. Again they tend to use fosterers rather than kennels and it can be a fabulous way to find an adult dog of a particular breed.