Well, I have a sibling and two children - so no advice here on what it's like to be or have an only child. What I would like to say is not to take too lightly the comments about it being harder work.
Don't get me wrong - I always wanted two kids, I didn't hesitate to have my second, I adore them both and now feel like my family is complete. They're only 2 and 4 but already play together, learn from each other, hug and kiss each other, amuse each other when mummy or other friends can't or won't, and yes, fight with each other too. I can't imagine life with only 1.
But I think it's fair to say that life with 2 children is more than twice as hard as life with one. People say the second one is easier and in some ways this is true - you certainly don't sweat the small stuff second time around. Dummies, good or evil? Didn't sterilise that spoon before DS stuck it in his mouth. Which pushchair/carseat/highchair to buy. You pretty much don't worry about it unless it's going to kill him. But unless your second is an absolute angel baby who never cries, never wakes up at night and never gets sick, you start to worry alot more about your own well being.
I think it's the levels of stress with more than one that seem to multiply. Imagine you're trying to get their dinner ready and they're hungry and don't want to wait. With one child - give her a breadstick or put the telly on for a few minutes. Problem solved. With two - the little one is screaming, doesn't like telly at the best of times and won't be appeased with a mere breadstick, the older one needs help on the toilet - or worse yet has had a little accident. Very quickly your blood pressure rises!
And when they get sick, they never get sick at the same time. So it's one and then the other (and then maybe you if you're lucky) adn then maybe the first one again. Which can mean long periods of time stuck at home/off work/nursing sick people. Just another thing to think about.
As I'm sure you know, children demand a lot of love and attention. And you want to give it. You want to be a good parent and give them your undivided attention for at least a little while every day. You want to hear about their days, see what they've learned, read to them, draw with them, teach them something new, or maybe just sit and cuddle them. And sometimes they don't want to share this time with a sibling. Which I think is fair enough. I mean, they do play together, and I do play with them both together adn stress the importance of sharing. But it's also important that they each get a little bit of you just to themselves. Which is fine except it means the amount of time just for yourself gets smaller and smaller.
Please don't misunderstand - I'm not trying to put you off. I love having two children and wouldn't have it any other way. But it seems that a lot of people seem to add on the "yes it is harder work but.." as a bit of an afterthought. I just thought you should give that side of it a bit more consideration.