Puppies, every time.
Puppies are work, but they turn into dogs, which are a great pet, in my humble opinion. Social by nature, they need effort to be raised correctly. A good dog is a great joy, but an unstable dog is downright dangerous. You do have to know something when you get one, whereas cats - so long as they had a reasonable youth - are all far more nearly the same. For example, housetraining. Cats fairly well train themselves, but many dogs do need work. The right dog will go hiking and fishing and camping with you and be, not a chore, but a joy. Further, where I am from, dogs get people to interact with each other. Dog parks (fenced areas where people let dogs off the leashes to run around like wild heathens) where I live are great places to meet people. Oh, if I only knew that when I was single. Good dogs are virtually always happy to see you and will rise to your level of activity: active when you want to be and passive when you want to be. If you lead a very active lifestyle, get the appropirate dog or unhappiness will ensue. Cats? Well, they have some personality and I love them, but they are not dogs. Further, good dogs are great personal and property protection and can be friend, companion and some degree of protection for your children.
All things worth having require investment of work, and some planing ahead of time. Although cats require little work or planning the rewards are limited. Your milage may vary, but I'd never live without a dog, perferably a large breed.
A Newfoundland (the dog on the "Big Dog" brand clothes, similar in size to the more familiar Sanit Bernard) inspired Lord Byron to write:
Near this spot
Are deposited the Remains of one
Who possessed Beauty without Vanity,
Strength without Insolence,
Courage without Ferocity,
And all the Virtues of Man without his Vices.
This Praise, which would be unmeaning Flattery
If inscribed over human ashes,
Is but a just tribute to the Memory of
BOATSWAIN, a DOG
Who was born at Newfoundland, May, 1803,
And died at Newstead, Nov 18th, 1808.
Although perhaps a bit over the top, especially in this day and age, you can get similar sentiment from many dog owners.