Dobbin,we have just one child - I started a thread on here a while back about only's! Our son is three and a half now and I guess my comments to you (from my slightly longer experience!!) are to BE TRUE TO YOURSELF and YOUR CHILD. My personal opinion is that a good 50% of second children are born to people who give it no thought at all, certainly not as much as you are giving your decision! So don't be swayed by thoughtless people who just do it because it's what you're supposed to.
There are good reasons for having two or more kids as put forward by the thoughtful sort of people on here so listen to them but make sure you listen to what your gut feeling tells you about what you want to do. I personally believe that there are huge benefits to being an only child - in receiving your parents time and attention, and in life being able to be tailored to you to a certain degree - but this kind of depends on your parents, some only children might not get the benefits if they have selfish parents!
However this is obviously not you! You have hit the nail on the head and realise that experience with other children is key. One thing I think I underestimated was the amount of socialisation that siblings do, every second of every day - so even the most socialised of onlies may be getting massively less experience of playing with other kids. I thought I'd done OK with this but I really think my son may be struggling at pre-school with this: when I am with him, he is actually ebullient and sociable, putting his cousins (siblings!) to shame and making them seem a little introverted!!!! However this at present does not seem to carry over to playing with other children when I am not there - HOWEVER, it's early days and I strongly believe he will be fine - I just want to give you a picture of how life is with an only.
As he's got older, it has been a real joy of my life to be able to study his little face as he's playing with me, or being read to or whatever - and I have really treasured the fact that I have not had to try to split myself in two as regards attention. I know I am meeting his needs, and helping him develop his full potential and I enjoy doing this 100%. It suits me and if you think you would feel too stressed by your time being split, then you probably WOULD! I think it's about knowing yourself.
Some only children posted before and somebody said something I will always remember, and that was about feeling TOO much 'in the spotlight', even as an adult. One thing I hadn't considered was the incredibly delicate balance that there is to hit, between giving your full time and attention, and making them feel smothered and like they can't do a thing without you commenting on it! I struggle with this.
One of the other things I believe passionately is that people think you're never on your own with a sibling, like when your parents die, etc. I just think this is wishful thinking. I have a brother I love like mad but on speaking about our childhoods, we experienced the SAME upbringing so very differently! And our relationships with our parents are sooo different - don't fondly imagine siblings will share everything. We are actually on our life journey alone and have to deal with things alone, when all's said and done.
So sorry to ramble but I just wanted to help if I can. Our son is gorgeous, articulate, an absolute delight and we have never regretted our decision. If you do stick with your decision, don't second guess yourself. Just be strong! Hope you enjoy it all!