Well, for starters, an 8 week old pup is going to be like having an infant in the house LOL whereas a 2 year old will still be very young and chipper but already have a personality in place, and probably be easier
There are pros and cons to both. I prefer puppies but that is only because my Jackson likes puppies and would adapt better to a dog that was young vs. another adult coming into his home. But I am also into training and dog sports and I kind of like to 'raise' a puppy, as hard as it is.
Young adults are great because what you see is what you get. The personality and temperment is pretty much already there and it's fantastic that you already know she gets along with kids and other dogs. Puppies are kind of a crapshoot, you really just never know.
As far as Yorkie vs. Havanese... Yorkies are terriers first and foremost. Some are the quintessential lap dog, but most will have lots of energy and have that 'terrier edge' to them. Havanese are probably a bit more laid back, in my experience, but most are known to be very lovable, good with kids and strangers, easy to train.