A puppy would be great, keep us fit and generally bring much happiness to ds 1 and ds2. I wouldn't have to give birth to it or feed it in the night. And I would be able to go for proper walks where we covered more than 50 yards in an hour.
On the con side it would eat shoes even more often than ds2 and I can see myself regretting only having the two kids.
I'm open to the possibility that the puppy thing is a technique for avoiding having to really think about having another baby, but this is the way my mind always seems to go when I think about it.