I asked CatFancy.com for some input on the subject, and here's the reply I received:

Dear Sharon,

Thank you for writing us at CAT FANCY!

According to "Cat Breeds of the World" by Desmond Morris, there are not many differences between calicos and tortoiseshells, besides their coloration. Both types of cats are almost always female, and the males are generally (but not always) sterile. The main difference seems to be related to behavior. Morris characterizes tortoiseshell males as acting like masculinized females rather than as males, meaning that they show little interest in asserting themselves against other males, mating with females, etc. The way Morris puts it, "So, although it is not true to say that all tortoiseshell cats are female, it is true to say that they are all feminine -- even the rare males."

I hope this information is of help for you.

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I don't know if I can infer from this that calicos (male and female) display more assertive behavior. I did write back and ask for a more detailed explanation of the differences in coloration, and I'll post the answer if and when I receive one. It's been my understanding, though, that a calico coat has three colors (black, orange & white or gray, peach & white), while a tortoiseshell coat only has two colors (black & orange or gray & peach).

Certainly the calico cat my family had when I was growing up, Rosie, was assertive; she ruled the rest of our cats with an iron paw. She was quite protective of her territory, and was known to chase large dogs off the property and, in one case, she actually rode a dog to the property line, hissing and digging her claws into the dog's back. No animal messed with Rosie more than once, yet she was always pleasant and affectionate with people.