I think there is no perfect one. If you wait until they are 5 you have to deal with school pickups.

If you wait till they are three, you get the "threenager" - and I can tell you - its frustrating.

If you wait till they are 2, they still need your attention and don't quite understand "shhh I'm putting the baby to sleep"...

So yes, pros and cons for every age gap.

I can tell you that I liked the 19 month age gap between DS and DD1. 16 months is still a bit too little, and 3 years is a nightmare. Or maybe thats just my 3 year old? LOL