I am an only child. Personally, as a child I loved being an only child. Had occasional dreams of having an older (haha... wasn't good at biology obviously ) brother but mostly was glad to have all the attention, my parents were able to send me to a good school etc.
I think it's important to make sure that an only gets lots of socialisation. I didn't as I was brought up in a village where my parents didn't mix much with local people & children. Even now as an adult I feel quite awkward in busy social situations. But then again, that's me, may be more down to personality than upbringing, and I'm quite happy I'm an introvert too!
As an adult though, I really do wish I had a sibling. Particularly when my dad died, I would have liked someone to share that with who had actually grown up with my parents, iyswim. Dh was ace, but it wasn't quite the same. As my mother gets older, I do find the thought of dealing with an elderly parent on my own quite daunting.
Basically, I think what I'm saying is that it's all swings and roundabouts. I wanted a sibling for ds, but I think if for some reason I couldn't have had one then I'd have lived with that and don't think ds would have suffered for it. If you both want another child then go for it. Don't go for another child just because you think you "ought" to though.
Believe me, this second pregnancy of mine has been TOUGH and there have been moments of ambivalence towards the second child already on my part. If I'd been pregnant out of obligation rather than really wanting another, then I think I'd be having serious regrets already.