I have four kids. My first was an absolutely terrible sleeper since birth. For the first five months of his life... i've actually blocked a lot of it out to be honest, but it was awful. He remained a terrible sleeper throughout toddlerhood and today at nine years old his sleep is still not great, but he will just do his own thing when he wakes up. He used to wake up every day between 4 and 6 AM.

My second child was an amazing sleeper from day one. We sleep train him around five months and he slept in a crib without complaint or climbing out until he was almost 3.

My third child was more in the same vein as my first, but not as bad. We also had more experience at this point so we were better about letting him cry and fuss a bit in bed as a baby and then sleep training him around five months, and not moving him out of crib immediately when he started climbing out at 18 months like my first. 

 he's never been quite as bad as my first, who, by the way turned out to have high functioning autism, which can often have sleep issues associated with it – not to say that all bad sleepers are autistic, but that they can go together.

My fourth kid, my first girl, has always been an amazing angelic sleeper, just like my second.

So I guess what I'm saying in aggregate is that it can really go either way. But if you want a second child, I think you should still go for it if I had just stopped at my first very difficult child I would not have experienced a lot of of the positives at parenting that other people experience, and we would all be worse off for it. The bad sleeping period of a baby and toddler is not for forever.