I'm sorry that this response is so delayed. I hope things have been ok since your last post in this thread.
It is tough to decide when to have kids. For my own experience, my husband and I did not plan either pregnancy, but we love and are thankful to have been given the opportunity to be parents to our kids. We have our darling little boy, who is 5, and our darling little girl, who is 2. The age split is a nice one since they are still close enough to be friends, but they are far enough apart that they do not have to share EVERYTHING!

I will make the arguement that only children are usually VERY lonely growing up. If the parent's are not there, they have to entertain themselves. When they are young, they are usually attached to the Parent's hip. I was an only child, and as an adult I still want/wish for a sibling. I"ve adopted my best friend from college as a sister since we are two only children together. When my parent's pass, I'll be the only one left of my nuclear family. That is a personal and somewhat morbid reason for siblings.

About the whole good parent to two vs. great to one thought - I do not think you can compare parenting of an only child to parenting of siblings. There are so many things that children with siblings know/learn/understand that only children do not. As an only child, I had the preconceived notion that the universe revolved around me because my parents only had me to worry about growing up. I've slowly broken that down over time and my transition to adulthood, but it still influences my actions at times. My husband, who is the eldest of three, does not have that notion in his programming. Granted, he is selfish with his time due to growing up and having to give so much time to his younger siblings, but he instinctively knows that he is not the center of the universe. Having a sibling allows the child to develop and learn differently than an only child. Maybe read up on birth order studies to get a better idea of what characteristics children born and certain times to certain family groups typically develop?
If you are really worried about the good vs great parent deal - maybe ask your daughter if she wants a sibling? I know I asked my mom constantly for a baby brother growing up. Discuss the idea of a second baby as a family, weigh the pros and cons.
I'm in favor of siblings, although economic factors will make it tough as well.
For my story, my husband and I have agreed that we can only afford two, especially since I want to get my doctorate. He's being super supportive and agrees that our third child will be my doctorate. That is the primary reason why I'm applying to be an egg donor, so my eggs won't go to waste while I am pursuing higher education.