Mums of two or more. How different was your second baby from your first?

My daughter is nearly 18 months and my partner and I are thinking about trying for another child later this year. I definitely want at least two kids but I have a silly little worry that won't go away! Basically, our daughter has always been what I would call a straightforward child. She slept through the night from 8 weeks, never had colic, never been ill apart from colds and never had any reactions to vaccinations etc. As soon as she started solids it was obvious she enjoyed proper food and she will try absolutely anything I give her to eat. She has never been fussy at all. She started walking and talking right on schedule and is generally a very happy and easy going little girl. The worry I have is that the next one will be the total opposite! I know it sounds silly but there is a more serious reason for this - I had PND for about 4 months after my daughter was born and I remember being very grateful at the time that my baby was an "easy" one. I am worried that I may not be able to cope with a more "difficult" baby if I get PND again. So, mums of more than one child - how different was your second baby to your first and did you find this difference hard to cope with? Obviously, I will love a new baby no matter what but it would nice to be a bit prepared!