Completely different really. Lee is part of a great big slot-in system which is expensive and currently the Big Stopper is on several weeks back order. If you want to get into grads and stuff, that's the way to go. HiTech Pro Stopper is a (cheaper) alternative.
Of the screw-in types, the new multi-coated B+W is probably the best, at a price, but otherwise I'd go for the LightCraftWorkshop ND500MC (from Premier Ink). In fact, I did - sold my regular B+W ND110 which has quite a strong orange cast (though they all have a bit of a cast).