What's the best kind of dog to get?

I have 2 kids, (4 and 2) and I am pregnant with twins. My husband and I think its a good time to get a dog (don't ask why, we just do). None of our kids are allergic and we are thinking that if we get the puppy before the babies come, they will become immune to the dander. I was thinking a small lap dog, but my husband wants a big dog (naturally!) If we were to get a big one, we would get a mastiff and if we were to get a small one, it would either be a pug or a shih tzu. Your opinions?