I'm a parent of two but an only child myself.
I remember that I liked being an only child when I was very young. But around high school age, I started seeing how many of my friends had close relationships with their siblings, and wished I had that. It was never a big issue, but ever since then, I've always wondered what it would be like to have brothers or sisters. I think it would be neat. Both of my parents had a number of siblings, so growing up I had a lot of aunts and uncles. My kids will not have any aunts or uncles on my side of the family, and my wife only has one sibling. The "big extended family" is another fond memory from my childhood, and one my kids will never know.
From the perspective of a parent, we had our second when our first was about 2.5 years old. Our first was an "easy" baby: slept a lot, virtually never fussed, no health issues, etc. Our second was and (to some degree) continues to be
very hard. She had colic and acid reflux, so for the first four months, from about 6pm until midnight, our choice was to constantly execute
all of the
5 S's or listen to her wail. She's six months now, and the evenings aren't as intense, but she keeps getting colds, which means the fussiness comes and goes and sleep is problematic.
Also, it was a double whammy because our first has had a hard time adjusting. She went from getting 100% of our attention to way less than 50% due to all the attention the baby required.
Some people may find this statement appalling, but for the first four months I often regretted having a second. I rarely feel that way now, but I still find myself looking back on when we only had one and how "easy" things were then.
In my experience, two kids isn't twice as hard as one, it's
ten times harder.
My wife and I knew from the start that we both wanted two kids, so I'm definitely going to get a "permanent modification". But if I could have seen into the future before conceiving #2, it might have changed my desire to have two. My wife doesn't feel the same.
At any rate, for now I'm just a complainypants, because both kids are healthy and thriving, and things should get easier at some point. But sometimes it's hard to see the light at the end of the tunnel when all at once you're completely sleep-deprived, the baby is screaming, and the toddler is getting into trouble.
Take any and all answers to your question (including mine) with a huge grain of salt, because there are so many dynamics involved, it's impossible to get any kind of meaningful advice.