Anise Seed vs. Fennel Seed with Taste as No.1 factor
I thought this was an interesting question, so I went into the kitchen and had a comparative chew. Anise Seed (or aniseed where I come from) is smaller, darker and gives a more immediate anise hit, but it fades quickly. Fennel seed is two or three times larger and paler. When you chew it, there's little flavour at first, but then a wash of anise flavour comes in. I think fennel tastes a bit softer, smoother, "greener" and more rounded.

If I had to substitute anise for fennel, I'd probably use a third or even a quarter of the original quantity. Although you might just want to leave it out. I agree with bikeboy - the great joy of fennel in Italian food is biting on fennel seed and getting that green-spicy hit in a meat dish (oh! I have a pasta dish with sweet Italian sausage, tomato, cream and fennel that is to die for). I don't think you'd get the same effect with anise.