Boy, I think Chris got it covered pretty well.
Now the only thing I would add at all is when you compare bows it really won't matter which system you use, just so you are consistent. IBO is the most commonly used now. But don't think that any bow you get will produce such speeds. The average hunting setup will normally produce something like 30-50fps less than any IBO ratings.
The only thing either of them is any good for is comparing one bow against another, and so on. If you want a faster bow after being set up, then you will have to start out with a bow that is faster to begin with----all things being equal.