My three girls are 5, 4 and 2 1/2
The first two are under a year apart (the younger one came home from the hospital on my oldest 1 birthday)
The youngest two are almost 2 year apart, and we are pregnant with number 4, which will come a few weeks before the baby turns 3.
Honestly, what was easieset for us, and I love the spacing was the year apart. They are like peanut butter and jelly, and just adore each other. Believe me they have there moments, but then all siblings do. I wish we had the pushed to have the other ones closer in age as well.
Yes, with the first two, there were times it was hairy, but I think adding any sibling there will be those times. Mine all potty trained really early (18 months or so), but honestly two in diapers so what, you are already in the thick of it, why not add another one. I love the crazyness of our house, I love the noise, the activity, I love it.