DDs are almost 5 years apart (they are 3 years old and 8 years old now), and it has been great. The older one had almost 5 years with us before the younger one came along, and I think that she is better off for it. I also liked having one baby at a time. I know that many people like the 2 year gap to get all of the baby stuff done at the same time, but not me!
I read once that closer together is easier for the parents, but 4 - 6 years is easier on the kids. For me, the 5 year gap was easier for me, too.
I really thought that I would have prefered closer to 4 years apart. When our oldest DD was 3, we started thinking about the second. We had a number of reasons why we couldn't start trying, so we ended up waiting until oldest DD was 4. The almost 5 year gap has been great, though.
As other posters have said, three years old is the magic number. Once they get to three, they turn a corner - potty trained, dressing themselves, playing by themselves, playing with playmates, feeding themselves (DD3 will get her own snack!), etc. That part will be easier on you if they are over three years apart.
Another thing to consider - your baby girl is probably pretty easy right now, esp. compared to the newborn. I found 6 months to be a wonderful age - cute, happy, sleeping on a schedule, etc. Things just don't seem to stay that way, and I wouldn't expect them to be the same by the time baby #2 comes along. I found that it goes through stages - newborns are difficult, infants are easier, difficulty comes around again when they start moving and walking, then they get easier when they start to understand limits, and then they start trying to exercise independence and say no to you (difficult again). And then they get easier again. I would just keep that in mind.
Good luck with whatever you decide!