As PPs said, there are SO MANY different types of olives, that it's impossible to really say that much about a difference between green and black ones generally.
If you have any great delis near you, you could go and get a few samples of different types to see what you like. Different olives are good for different things, too, and you'll quickly figure out what you prefer.
My local greengrocer's grandmother processes olives, and they sell them in the shop. I'm not sure what olive they are, but they're unreal! And black.