I have to agree with Heather it is a very personal decision. One that is often looked down on by others, even though it really is none of thier business.
My Engineer husband and I have only one child, we originally thought we would have two. However, we came to the conclusion shortly after our amazing daughter was born that one would be right for us. She will even tell you she likes being the only one to snuggle with her parents, but she will also tell you she enjoys the company of other children.
I have always had her around other children, but one huge advantage is that she can play with others or independantly. From my expirenece I can not say that has always been the case when we are around family members that have siblings.
I grew up in a household of 3 biological daughters and 45 foster kids (never more than a couple at a time). However, by the time I was in second grade I was the only one left at home. I turned out fine, basically an only able to solve my own problems, not rely on others but can work in groups and with a team just fine.
So I think as long as you do not hide your child away an only child can be fine.
I have seen some other posts from mom's on this subject in the past, who will tell you your child will be a brat because of it. However, I tend to feel that is all in the parenting and thier are plenty of kids who are brats that have a sibling or two. So do not let that sway you, because it is not a very true statement.
I recently read a book on the subject, something about rasing an only. It did have a chapter about making that decision so it may help you and your spouse out a lot.