You can look at either one as having positives, which actually can both be turned into negatives if you flip the side of the coin.
I have a mare, but she is very laid back. The general consensus I hear is that geldings are quiet, but too quiet for some people, those people usually like mares. Mares can have more "try." It's why you see so many more polo pony mares than geldings. They'll run themselves into the ground when a lot of geldings go "meh." However, as some people have said, each horse is different. Stereotyping horses, or lumping them all into one category, will only get you so far and is certainly not fool-proof.