Just wanted to add my two cents worth!
As some other posters have said, I dont' think there is an 'exact' right time to have another child. Reilly was 3 and 2 months when Georgia arrived...and I was also a bit anxious and worried how we'd both go when she arrived. It had always just been 'him and me'...and he was my 'baby'...I wondered if and how COULD I love the new baby just as much as him..would he be left out etc...would I cope with two..COULD I cope with two?
In the end..all the worrying proved to be a waste of time. Georgia is such a quiet placid little baby and has been from birth and Reilles just adores his little sister. He was a bit jealous and a bit funny with me in hospital, but I guess that's normal. From the second day, he wouldn't leave her side. He's the proud big brother. When Georgia is sleeping in the day, that's 'mummy and Reilly' time. We colour, read, play...whatever he wants to do. Sometimes on weekends, I take Reilly out to a cafe for a milkshake, just 'Reilly and Mummy'...or I take him to the movies, something special for just the two of us. When Georgia is old enough, I will do the same with her. Paul also takes him out and has 'Reilly and daddy' time.
I am also thinking about TTC towards the end of the year, and the same little fears and worries are setting in as well, but I'm not so concerned by them this time. New bubs will slot into the family just fine...and so will you! It's perfectly normal to have these worries..so don't stress too much!
I think in a way it's easier to have them close together, as somene said, Jacob will only ever remember having a brother or sister.
Hope my 'essay' helped somewhat!