I ended up with a 2-for-1, so I can't tell you about having one child from a parent's perspective. However, I am an only child myself, so I can give you my thoughts from that angle.
I agree with the things others said. The financial aspect - they were able to give me what I needed and most of what I wanted (within reason). They were able to devote their attention to me. There was a whole lot less bickering.
I would imagine that I have a closer relationship with my parents because it was just the three of us. Hard to say that for sure, as it might have happened anyway, but probably not as much.
I think only children tend to be creative too, as they have to find ways to entertain themselves. I noticed that a lot with my Dstepson, who was an only child until the boys were born, as he could turn just about anything into a toy.
I love being an only child, but there have been times where I wished I had a sibling. That's especially true as an adult, as I see the close relationships that many people have with their siblings in adulthood. Of course, this doesn't always happen. And as a kid, I never had a shortage of people willing to "sacrifice" by donating their brother or sister to me.
Honestly, knowing that having a sibling would totally change who I am, I wouldn't have it any other way than the way it is.
There are pros and cons either way. Talk to your DH and follow your heart. If you make the decision together, you'll be happy with it no matter which way you go.