I do not agree with either side in this debate. Instead, I firmly believe that both sides have some truth to them. We are surely born with some tendencies that shape our lives. At the same time, there are clearly ways in which our upbringing affects how we actually turn out.
As an example of how nature affects us, it seems clear that different people are born with different personalities. This is particularly easy to see when you look at siblings. It is common to have one sibling who tends to be more serious while another is more easy-going. One can get along with others better than another. Parents often notice these things when the children are still quite young.
On the other hand, nurture clearly affects us as well. For example, if parents pay a lot of attention to a child and interact with that child in many ways, the child is likely to do much better in school than a child whose parents leave it alone and let it watch TV all the time.
Therefore, I do not think that I could ever take one side in this debate. Both sides clearly affect how we turn out.