I have siblings, but like you, they are way older than me. And so we never really spent a lot of time together in that regard. They did however babysit me a lot when I was very young, and I only have fond memories of that. Being an only child also means it was less of a financial burden for my parents to raise me at the time, meaning I got spoiled. I never felt like I needed anything. Ever.
When I was younger, I did dream of how it would be like to have a brother around my own age, so that I always had someone to play Playstation/PC with at the time. I even talked this over with my parents at one point, and I immediately got the response: "But then you would have had to share everything with him." Needless to say, young me wasn't up for sharing jack shit, so I dropped that idea pretty quickly.
I never really felt lonely, because for the most of my childhood I've had friends that lived very close. I could either walk/bike to their house and they could do the same.
I do however feel like that siblings share a really tight bond in regards of friendship. I mean, how could it be anything else? You live together for the most of your childhood after all.
With all of that said, my point of view in this thread will be of the assumption that we are talking "growing up as siblings", taking a younger point of view. I.e. early childhood through teenage years. This is important to note, because obviously ones adult relation to their siblings is going to be different.
Not sure what you meant in your headline when you said "Only Child VS. Siblings". You want pros/cons?
I can give you some pros and cons on being an only child I guess.
Pros about being an only child would be,
*Your parents focus solely on you, and you alone. (this can be broken into so many points more, but yeah)
*Also, when you felt like being alone, you could be. There's no sibling that can come around and bug you 24/7.
I can give more pros, but those are more of a negative connotation. Like,
*No sibling rivalry. etc.
Cons about being an only child would be,
*Not having someone to share your deepest secrets with. I know that when you have a really good friend, you talk about things. But in my experience, there are things you don't even tell your friends. Young age or not. I feel like if I was talking with a sibling around my own age, that would be different.
*Not having a playmate available in literally the same/next room.
I guess that's basically what I have on the matter.