None. Get the CR-3 - you won't regret it. Although Jeppesen don't make them any more, you can still get them on fleabay and ASA have a circular version of the E6B which is the exact same thing. It's also less than half the price. If you feel you must get a slide driven flight computer, get the AFE ARC-2 - it more or less the same price and is much better made.
Phil