I would never have just one dog if I could help it.
Two together make it easier on me and on the pups. They have another one to sleep, eat and play with, especially when they are home without owners. Even with different personalities, they become attached to one another. I would think the financial end of it should be examined though - 2 times the vet, grooming, licensing, food, etc. can add up.
The hardest thing is if you have just 2 and one dies and the other is left to grief alone for the one they lost. That's another reason I support what some have eluded to - have 2 that are similiar in age. There are no guarantees that they will have similar life spans, but chances of similar life spans will be much closer if they are about the same age, rather than if they are several yrs apart. Of course, if you're going to put a big dog with a little Yorkie the big dog might actually have the shorter life span. Not sure why little dogs live longer lives - maybe because we owners baby them all the more!
yorkiesmiles Bubba and Roxie