To tell you the truth, the only one who can determine good spacing is yourself and your husband. There are benefits to spacing them out more, and benefits to having them closer together. You have to do what you think is best for your family.
My oldest two are 18 months, 4 days apart. When they were younger, they were each other's best friend and worst enemy rolled into one. They still have some of the same closeness, but nowhere near as much. They share some of the same friends, which can be good in that they usually get invited to the same things. On the other hand, there are days that only one can go (for example, sleepovers) and the other gets upset that he/she is left out. The fighting can be awful now that they are older, too, but when they get along they are still close.
On the other hand, my youngest (#3) is five and a half years younger than my second child. He is not as close to his brother and sister as they are to each other, but they enjoy having him around (most of the time). I loved having the one on one time with him while brother and sister were in school, something that I didn't get to enjoy as much of with my older two. With my daughter, I only got to enjoy her one on one for the first 18 months of her life. With my son, we had one on one time for the first two years that my daughter was in school. With my youngest, I'm getting a full four and a half years of one on one time while the older two are in school. On the other hand, I don't get to chaperon field trips or volunteer in the classrooms as much as I used to. There are times that the older two want to do fun activities that we can't take their younger brother to, and we can't always find someone to watch him if Daddy is at work or if we want Daddy to go with us.
Like I said, there are pros and cons to all situations, no matter how you choose to separate them. Only you can know which is best for you. Good luck!